We have re-examined the formation temperature of carbonates in ALH84001, accounting for the prior evolution of the stable isotopes used in the analysis. Oxygen and carbon in the atmosphere likely have become isotopically fractionated due to various nonthermal loss mechanisms operating throughout geologic time. Taking this previous evolution into account, we derive a formation temperature from oxygen isotope thermometry of 40--250 ¿C. This range of temperatures represents a lower limit; an increase in groundwater salinity could further raise the Δ18O of the groundwater, as seen in terrestrial hydrothermal systems, allowing for an even higher temperature of formation derived from the isotopic data.¿ 1997 American Geophysical Union