Garnet is an accessory mineral in the Cape Granite Suite, and garnet δ18O values in the Peninsula Granite range in from 10.0 to 11.4‰ (mean 10.6 ± 0.6‰, n = 15). These values are consistent with the garnet being produced during incongruent melting of a metapelitic source that has a similar O-isotope composition to the Malmesbury Group. Peninsula Granite quartz δ18O values range from 13.2 to 14.0‰ (mean 13.6 ± 0.3‰, n = 17), at the high end of the range previously observed for the Cape Granite Suite. These high δ18O values are consistent with the source of the Peninsula Granite magma having a greater component of clay minerals, which have inherently high δ18O values. Garnet has a high closure temperature (>800 oC) to oxygen diffusion and its δ18O value should, therefore, correlate closely with that of the source. Quartz has a significantly lower closure temperature (~550 oC) than garnet, and sub-solidus oxygen isotope re-equilibration between quartz and feldspar during slow cooling ought to result in a greater variation in quartz δ18O values compared to that of garnet. That the reverse is the case suggests that granite magmas were derived from a moderately heterogeneous source, as expected for metasedimentary rocks. This source underwent melting to produce different batches of granitic magma containing entrained garnets of slightly different δ18O value. Magma batches were subsequently mixed and homogenized before and/or during the emplacement process, resulting in a narrower spread of quartz δ18O values.