The thermodynamic equilibrium between sillimanite and andalusite has been redetermined from new heat-capacity measurements on andalusite and different samples of sillimanite. Drastic differences are found for bulky “sillimanite” and “fibrolitic” material. Whereas the phase diagram of “sillimanite” is in agreement with the experimental brackets of Holdaway (1971), that of “fibrolite” is closer to the results of Richardson et al. (1969). The enhanced heat capacities of fibrolite as compared with sillimanite are discussed in terms of possible lattice faults in fibrolite that are dominant relative to the role of metallic impurities, Al-Si disorder, or nonstoichiometric Al/Si ratio in the samples investigated.