The optical properties of two tetraphenylporphyrin derivatives based on tetra(4-aminosulphonyl) phenylporphyrin have been examined. These porphyrins can exist in both free-base and dication states with their Soret absorption band occurring near 420 and 440 nm respectively. Floating monolayers can show the occurence of a new state characterized by two additional bands near 490 and 700 nm. The formation of this unknown state is dependent on both the subphase pH and the area occupied per monomer unit at the air-water interface. The effect of the subphase pH on the production of this state has been investigated systematically for both porphyrins and comparisons between the two habe been made. The optical absorption of Langmuir-Blodgett films for one of the porphyrins has also been examined. This has shown that the unknown state can be transferred on to glass substrates, though it is found not to be stable if stored in air.