In their very interesting article in this journal, Higgins and Kerrich describe how loss of C02 from a C02-H20 mixture will normally depress the SI80 of the total mixture. This is obviously the case, since CO, is ''0 enriched. When the fluid as a whoie gets depleted in C O ~ , it therefore also gets depleted in 0. The above authors also present temperature evidence from fluid inclusions as well as S1'O analyses of quartz crystals. From these data they calculate 6180 of H20 by means of a quartz-H20 fractionation curve. They find a history of decreasing 6O values of the H20, which they call sometimes H20 (Fig. 2), but, in their main argument, Although loss of C02 lowers the S1'O of the remainder of a diminishing hydrous fluid, it does not need to change the S1'O of the H20 in the fluid. Contrary to the case of evaporation of H,O, the lost fraction of C02 is not being replenished from the body of the H20, unless additional species, such as CH, or graphite, act to buffer the C02 loss chemically. In the case of mere C02 loss from a binary, isothermal, unbuffered mixture with H20, the manner of loss of C02 (batches or infinitesimal steps) has no effect, for the same reason: the components are not chemically interchangeable. A quartz-H20 fractionation factor can be used to calculate 6180 of the H20, as done by Higgins and Kerrich, but the value obtained is unrelated to presence or absence of C02 at constant temperature, unless the mixture is chemically buffered. Taylor (1974) has suggested a range of 6' values for magmatic H20, not magmatic Of direct interest is, however, the fact that of a C02-H20 mixture will depress the 6180 of the H20 component, in proportion to the relative amounts of the two species present (which should in that case be measured in atom percent of oxygen). This could be of some significance at Grey River and elsewhere, but it depresses the S1'O of H20 most, if C 0 2 continues to be present in as large a proportion as possible. In the case described by Higgins and Kemch the would be very much smaller than the they suggest follows from C02 loss. For example, the continued presence of 33 mol% of C02 could shift the S0 of the H20 by less than 1%, between 400 and 350°C. A graph showing this cooling effect quantitatively is shown in Fig. 1, valid for a binary C02-H20 fluid with an aggregate 6180 of O/,,, in a closed system. If the isotopic of C02 on H20 is to be modelled for a geological situation, both the origin and original S1'O values of the water and C02, and the whole subsequent temperature and transfer history must be taken into account. A major part of the significant drop in the deduced 6180 of the H 2 0 at Grey River, originally postulated by Higgins and Kemch (i.e., 5/,), would obviously have to be due to causes