Decades of various and numerous isotopic studies to interpret the environmental con- ditions of dolomite formation proved to be inconclusive because the temperature- dependent oxygen isotope fractionation factor between dolomite and the solution from which it precipitated could not be determined experimentally at low temperatures. With the discovery of bacteria that mediate the precipitation of dolomite, it is now possible to overcome kinetic barriers and precipitate dolomite under controlled temperature condi- tions in culture experiments. Herein we report on the results of microbial experiments that have enabled us to calibrate the dolomite-water oxygen isotope fractionation factor and provide a paleothermometer to evaluate conditions of ancient dolomite formation. The temperature (T) dependence of the fractionation is defined by the equation: 1000 ln adolomite-water 5 2.73 3 106T 22 1 0.26.