Dynamic optical nonlinearities in free base tetrapyridylporphyrin (H₂TPyP) solutions were investigated at 532 nm with the Z-scan technique. We observed a reverse saturable absorption process that was found to have a fast contribution related to the singlet population and a slow accumulative contribution arising from the triplet population. The optical excitations and subsequent relaxations can be interpreted with a five-energy-level diagram that allows determination of the excited states photophysical parameters such as triplet and singlet absorption cross-sections and the intersystem crossing time.