Two contrasting types of Palaeozoic granitoids are of widespread occurrence in SE Australia and can generally be distinguished by their chemistry, mineralogy, field relations and initial strontium isotope ratios. Chappell & White (1974) have proposed that the granitoids are derived by partial melting of two different types of source materials: (1) igneous or 'I-type', and (2) sedimentary or 'S-type'. Oxygen and hydrogen isotope compositions were measured on 63 whole-rock samples of both fresh and altered granitoids and xenoliths. Analyses were also made of representative mineral separates from these rocks. For 'S-type' plutons, average δ 18O values range from 9.9 to 10.5, whereas for the 'I-types' the range is 7.9 to 9.4. Xenoliths are about 1 per mil depleted in 18O and generally enriched in D relative to the host rocks. The average δD values (and water contents) are - 62 ± 4 (1.10%) and - 77 ± 12 (0.73%) for 'S' and 'I' granitoids, respectively. Individual δD values range from - 50 to - 102 and correlate well with water contents: the more water-rich the granitoid the greater the deuterium content. Per rail 18O fractionafions between quartz and biotite are rather large ranging from 5.0 to 6.9 (independent of granitoid type), with a mean of 6.0. Thus typical isotope temperatures of about 520°C are inferred and retrograde effects are indicated.