The phenomenon of protonation of phthalocyanines (Pc) and its effect upon their photophysical properties has seen considerable neglect in the literature. The work reported here clearly shows that tetrasulfonated zinc Pc, a known photodynamic therapy (PDT) agent, is strongly susceptible to protonation at the azomethine bridges. Absorption and fluorescence spectra demonstrate the absolute dependence of the redshifted peak on the pH of the solution. The fluorescence spectra and lifetimes of the protonated Pc are reported, and the potential application of this phenomenon to the development of a PDT agent with increased selectivity is discussed.