Using IR radiation from a synchrotron source, high-quality absorbance spectra were obtained from polycrystalline powder of chloritoid (cld) from ambient conditions up to pressures of 10 GPa over 50 to 4000 cm−1. The idealized chemical composition of the chloritoid group is M2Al4O2(SiO4)2(OH)4 where M = Fe or Mg in our experiments. All of the 42 expected fundamental IR modes were observed. These data, combined with the response of the IR bands to substitutions of Fe for Mg, and of D for H, constrained the band assignments. Heat capacity (CP) and entropy (So) for the triclinic and monoclinic polymorphs of Fe- and Mg-cld were calculated from the Kieffer-type model, using our detailed band assignments. The calculated heat capacity and entropy for the monoclinic and triclinic polymorphs differ negligibly. The results at temperatures above 298 K are described by the following polynomial expressions in J/(mol·K): CP = 7.835 · 102 − 5.170 · 103T−0.5 −1.648 · 107T−2 + 1.934 · 109T−3 for Mg-cld and CP = 7.848 · 102 − 5.185 · 103T−0.5 − 1.548 · 107T−2 + 1.783 · 109T−3 for Fe-cld. At room temperature, Ss = 293 J/mol·K for Mg-cld and 335 J/mol·K for Fe-cld. These values differ somewhat from entropy estimated from various internally consistent databases (−3 to −9% for Mg-cld and −9 to +5% for Fe-cld). However, using our new So and CP values in conjunction with the enthalpy of formation, Hf = −7101 kJ/mol for Mg-cld or Hf = −6422 kJ/mol for Fe-cld (estimated in this study), we can closely reproduce the experimental data for the reactions Mg-chloritoid + talc = clinochlore + kyanite (Chopin 1985) and Fe-chloritoid = almandine + diaspore + water (Vidal et al. 1994).