A semi-detailed geological mapping program has been completed in the Billeroo South area of the NW Olary Block, Curnamona Craton, South Australia. Unlike other areas of the Olary Block, this area had not been previously mapped in any detail except for the central portion around Glenorchy, Mount Victoria uranium mine and the very eastern portion around Billeroo Huts. Existing maps of the Glenorchy-Mount Victoria area by Esso Australia Ltd in the late 1970s were scanned and digitised to provide a preliminary base for field mapping which was undertaken from late April to early June 2001. A preliminary solid geology interpretation of aeromagnetic data was also undertaken prior to field mapping. Field mapping was undertaken at 1:25 000 scale on digital orthophoto base maps provided by PIRSA. These base maps provided an excellent base for field use and subsequent GIS photo interpretation and map compilation. Combined with GPS readings of field locations, they proved to be very accurate and reliable. Lithostratigraphic rock units recognised in the map area were similar to those in other parts of the Olary Block, and easily fitted into existing nomenclature schemes. Mapped units consist mainly of a series of migmatitic metasedimentary rocks and granitoids. The metasedimentary sequence ranges in composition from quartzofeldspathic to pelitic, with calcalbitites, calcsilicate rocks, massive to layered plagioclase-rich (albitic) rocks (albitites), plus rare gossan and ironstones. Stratigraphic units are the Wiperaminga Subgroup and Ethiudna Subgroup of the metasediment-dominated volcano-sedimentary Curnamona Group, and the lower portion of the Saltbush Subgroup of the psammopelite and pelite-dominated Strathearn Group. The central portion of the Billeroo South area is dominated by large, voluminous intrusive granitoids of variable composition, exposed as semi-circular structures and flanked by highly migmatised, mostly psammopelitic metasedimentary rocks. These granitoids cover ~30% of the total surficial outcrop area, and are predominantly monzogranitic in composition. Small volumes of more mafic intrusive granitoids, ranging in composition from granodiorite to quartz diorite to diorite, commonly occur either as dykes and small intrusive bodies at the margin of the larger granitoid bodies, or in complex shapes within these granitoid bodies. Unlike the Crockers Well region, the Billeroo South area contains only minor volumes of trondhjemites. There are several generations of pegmatites. Late mafic dykes cross-cut both the metasedimentary sequence and the granitoids, but are veined by late pegmatites. The entire sequence, including pelitic rocks of the Saltbush Subgroup, has been subject to a complex multiphase deformational history, high-grade metamorphism, granitoid emplacement and migmatisation, which, in the central portion of Billeroo South, has disrupted and obliterated most sedimentary features. Consequently, stratigraphic relationships are generally ambiguous. Distal to the granitoid-dominated terrain, migmatisation is less intense: the lower quartzofeldspathic portion of the sequence is best preserved in the west around Black Hill and Jagged Rocks, whereas the upper pelitic portion is preserved in the east near Billeroo Hill. Three major deformational phases have been recognised in the Palaeoproterozoic rocks of the Billeroo South area. All three events developed strong deformational fabrics and intense folding at all scales, resulting in a complex multi-phase fold pattern. Pervasive, ductile deformation at peak metamorphism during D1 and D2 coincided with widespread regional migmatisation during D1 and a second more localised phase of migmatisation and anatexis during D2. The latter is associated with larger volumes of anatectic melts. Subsequent D3 deformation is characterised by strain partitioning and retrogression, and coincides with granitoid emplacement. Overprinting D1, D2 and D3 structures are well preserved in the Billeroo Huts area. Based on the field mapping program, the preliminary solid geology interpretation was substantially revised using a combination of total magnetic intensity, first vertical derivative and radiometric images. It was difficult to distinguish the various granite-migmatite types because aeromagnetic textures were similar across most of the region. However, the areas of intense granite formation and intrusion are clearly defined by the RGB composite radiometric imagery.