The acidity constants, as pK(a) values, of six monoazo resorcinol derivatives were determined by the UV-visible spectroscopic technique at 25 degrees C (+/- 0.1 degrees C). The protonation and deprotonation behaviors of the studied molecules were studied from the super basic to the super acid region (i.e., 10 N NaOH to 98 % H2SO4) including the pH region. It was observed that deprotonation occurs at the resorcinol ring. The first protonation occurs at the nitrogen atom next to the resorcinol ring, whereas the second protonation occurs at the nitrogen atom next to the phenyl ring.