The acid dissociation constants, as pK(a) values, of eight drug precursor 6-acylbenzothiazolon derivatives were determined using UV-vis spectroscopic technique. The protonation and deprotonation behaviors of the investigated molecules were studied from super basic to super acid regions (i.e., 8 mol.L(-1) KOH to 98 % H(2)SO(4)) including the pH region. It was observed that protonation occurs only at the amid group in the super acid region. By validating the obtained acid dissociation constants, elucidation of the structure and protonation mechanisms of the studied molecules was attempted.