Several generations of carbonate minerals, kaolinite, and quartz are present in Australian bituminous coals. Petrographic observations show that disseminated, fine-grained kaolinite and quartz are common in the coal. On the basis of oxygen isotope ratios, it appears that the disseminated kaolinite and siderite nodules were precipitated from Permian pore water early in the deposition of peat at calculated temperatures of 16 degrees to 32 degrees C. Secondary cleat-filling kaolinite at Leichhardt colliery in the Bowen Basin has a constant delta 18 O value of around +6 per thousand , which is consistent with formation from mine water of the present isotopic composition at a temperature of 56 degrees C. Lower temperatures are indicated for secondary kaolinitization in coals of the southern Sydney Basin. Detrital quartz grains and chemical quartz have similar isotopic compositions and therefore cannot be distinguished isotopically. However, the oxygen isotope ratio of disseminated, fine-grained silica is consistent with formation in the brown coal stage at a temperature of 52 degrees C and depth of about 700 m. Secondary carbonates are not readily characterized in terms of isotopic criteria and are best recognized by their mode of occurrence. The ability to distinguish between primary and secondary kaolinite appears to hold promise as a guide to prediction of outbursting conditions in underground coal mines.