We report the application of pressure perturbation calorimetry (PPC) to study unfolded proteins. Using PPC we have measured the temperature dependence of the thermal expansion coefficient, alpha(T), in the unfolded state of apocytochrome C and reduced BPTI. We have shown that alpha(T) is a nonlinear function and decreases with increasing temperature. The decrease is most significant in the low (2-55 degrees C) temperature range. We have also tested an empirical additivity approach to predict alpha(T) of unfolded state from the amino acid sequence using alpha(T) values for individual amino acids. A comparison of the experimental and calculated functions shows a very good agreement, both in absolute values of alpha(T) and in its temperature dependence. Such an agreement suggests the applicability of using empirical calculations to predict alpha(T) of any unfolded protein.