The nonlinear absorption of two porphyrin diacids (H4TPP2+ and H2DSP2+), the diprotonated forms of free base tetraphenylporphyrin (H2TPP) and dithiaporphyrin (DSP), were studied in the wavelength range of 500-650 nm. The two porphyrin diacids exhibited perturbed static and dynamic characteristics and enhanced nonlinear absorption properties relative to their parent neutral complexes in solution. Furthermore, for the dithiaporphyrin diacid, the introduction of S-atoms into the porphyrin core makes it a better candidate for optical limiting relative to the simple porphyrin. Their photophysical parameters such as ground and excited states absorption cross-sections, together with fluorescence lifetime and intersystem crossing time, were determined.