The thermal decomposition of boron–hydrogen compounds is complex and multistep and involves the formation of various intermediates. An accurate description of the thermodynamics of the reactants, products, and intermediates is required for an in-depth understanding of their reactivity. In this respect, we have proceeded to the accurate determination of the key thermodynamic functions (ΔH(T), S(T), and CP(T)) of 44 isolated B–H molecular species involved in the decomposition of B–H solids, with the inclusion of anharmonic effects. An excellent agreement is observed with available experimental data. We report the analytic expressions of these functions obtained by fitting them with NASA functions in the 200–900 K temperature range. Because the vibrational spectra of these species are their fingerprints, we also report the predicted IR and Raman spectra. The calculated anharmonic spectra show an excellent agreement with experiments and allow for a clear-cut identification of fundamentals, combinations, and overtones.