In the reactions of 3-hydroxy-1,2-benzisoxazole (BIO) and its 5-Cl and 6-Cl derivatives with ethyl bromoacetate were obtained the mixtures of O- and N- substituted esters but 7-NO2-BIO formed only O-ester. The products of N-aminomethylation of 5-NO2-BIO were active against Trichomonas vaginalis. The structures of pure compounds have been confirmed by elementary analysis and IR spectrometry. Determinations of dissociation constants (pKa) of BIO, 5-Cl-BIO, 6-Cl-BIO, 7-Cl-BIO, 5-F-BIO, 5-NO2-BIO, 7-NO2-BIO by potentiometric titration method were described. Investigated compounds have two values of ionisation constants except the nitro derivatives, this is the confirmation of equilibrium 3-hydroxy-1,2-benzisoxazole <==> 1,2-benzisoxazolinone-3.