Microspecies involved in prototropic equilibria of isoxicam, piroxicam, meloxicam, tenoxicam, and lornoxicam are characterized by time-dependent density functional theory (TD-DFT) and UV-vis spectrophotometry, their relative Gibbs free energies in solution and Maxwell–Boltzmann populations are theoretically calculated. TD-DFT calculations of the most intense transition energies for the microspecies involved in equilibria are compared with those obtained in solvents of different dielectric constants (1,4-dioxane, ethanol, water, and formamide) and at various pH conditions. Our TD-DFT results allow establishing the predominant oxicam microspecies at the experimental conditions. Geometry optimizations are performed at level of theory PCM-PBE0/6-31+G(d, p), and transition energies are computed using the protocol PCM-PBE0/6-311++G(d, p). © 2012 Wiley Periodicals, Inc.