Gerlich has shown that Sheard's model for calculating mode &ggr;'s from hydrostatic pressure derivatives of the elastic moduli of hcp Mg and Cd yields Gruneisen &ggr;'s at both high and low temperatures that are in good agreement with the &ggr;'s derived from thermal‐expansion measurements. For hcp Ti and Zr, however, large differences arise, primarily from very small values fordC44/dP. It is proposed that these small values are caused by the changes inc/aratio with hydrostatic pressure because of a large dependence ofC44on thec/aratio. The disagreement with thermal‐expansion data can be removed by taking into account the difference ind(c/a)/dVbetween hydrostatic‐pressure and thermal‐expansion conditions. The effect of &Dgr;(c/a) is not found in tetragonal TiO2, rutile, where&ggr;¯His in excellent agreement with the thermal expansion &ggr;∞.