AbstractAcidity constants at 20° C and the ionic strength μ = 0,1 were determined for 2‐benzenesulfonamido‐pyridine, 2‐benzenesulfonamidopyrimidine, 2‐sulfanilamido‐pyridine, 2‐sulfanilamido‐pyrimidine, 2‐sulfanilamido‐4‐methyl‐pyrimidine, 2‐sulfanilamido‐4,6‐dimethyl‐pyrimidine, 4‐sulfanilamido‐2,6‐dimethyl‐pyrimidine and 2‐sulfanilamido‐thiazole. The effects of substituents on the pK values are discussed. Thedata may be explained by the existence of tautomeric forms fo