The protonation constants of the amino nitrogens of some substituted 1,2,4,5-oxadiazaboroles have been determined in acetic acid by means of potentiometric titration with perchloric acid. pK(a) values of the title compounds were interpreted on the basis of structural effects due to the substituents and the main skeleton by plotting pK(BH)(+) value versus Hammett constants. Good correlations between pK(BH)(+), sigma constants, and delta(CN) and delta(NH) values were also obtained.