A partial phase diagram characterizing the conformational change that occurs in Thermomyces lanuginosus xylanase as it is slowly heated in 150 mM sodium phosphate (pH = 7.0) has been constructed from slow-scan-rate differential scanning calorimetry measurements. The Clausius-Clapeyron equation was applied to determine an associated volume change of −205 L•mol −1 at 24˚C, the equilibrium transition temperature at 1.0 atm pressure. This value is in excellent agreement with that predicted using a previously published [1] empirical equation for calculating the hydrodynamic radius if the transition is regarded as from a random coil to a functional, folded state and with the assumption that the hydrodynamic radius is a good approximation of the true random coil radius. The existence of a low-temperature random coil is confirmed by circular dichroism and dynamic light scattering measurements. Thus, at 24˚C and 1.0 atm pressure the enzyme appears to fold from a random coil to a functional, folded form as it is slowly heated.