Differential scanning calorimetry, pressure calorimetry, and densitometry have been employed to study the relation between volume and enthalpy changes in the melting regime of lipid membranes. We demonstrate a rigid proportional relation between volume expansion coefficient and heat capacity. This result is first shown in densitometric experiments. It implies that calorimetric profiles obey a simple scaling law for the temperature axes in experiments with applied hydrostatic pressure. In a theoretical paper (Heimburg, T. Biochim. Biophys. Acta 1998, 1415, 147-162), we have argued that this relation has far-reaching consequences for the predictiblity of elastic constants from the heat capacity. The proportionality constant between volume and enthalpy changes is found to be independent of the lipid, which has interesting consequences for the calculation of the elastic constants of biological lipid mixtures with unknown composition. We demonstrate this for lung surfactant, which displays a similar relation between volume and enthalpy changes.