Theoretical calculations have been made, with the aid of the Debye model, of the differences in molar volumes between different isotopic forms of simple crystals. Where comparisons are possible, the agreement with experimentally determined (X-ray) volume differences is fairly good. The theory has been applied to predict the differences between the molar volumes of the 32S and 34S forms of the minerals sphalerite, ZnS, galena, PbS, and pyrite, FeS2, and hence to estimate the possible influence of pressure on the partitioning of sulfur isotopes between pairs of those minerals. This partitioning has been used in the past as an isotopic 'geothermometer', with an assumption that it depends upon temperature but not pressure. It turns out that for the conditions that generally apply in practical isotope geothermometry (pressure ≤ 800 MPa, temperature ≤ 1100 K), the calculated pressure effects are considerably less than the uncertainties in experimental fractionation factors.