The protonation of 3-amino-5-methylisoxazole, 5-amino-3,4-dimethylisoxazole, sulfamethoxazole (N-sulfanilamido-5-methyl-3-isoxazole), and sulfisoxazole (N-sulfanilamido-3,4-dimethyl-5-isoxazole) in aqueous sulfuric acid was studied. The pKSH+ and log SH+/S or log SH2+2/SH+ values were determined by UV spectrophotometry. The data were treated by the acidity function method and by the method of linear free energy relationships. The comparison of the correlations obtained and the basicity and the UV spectra of the four compounds indicate that the process of protonation is the same in the amino and sulfonamide derivatives.