The dependence of the ionic forms of haematoporphyrin(LX) dihydrochloride (HpdiCl) on solvent composition was investigated. In 2.8 x 10(-4) M solutions of HpdiCl in apolar (C6H6) and polar (CH3CN) solvents, HpdiCl exists in dicationic form. In hydrogen-bonding solvents, such as CH3OH, HpdiCl can exist in neutral, monocationic and dicationic forms. In C6H6-CH3OH solvent mixtures, the ionic forms in which HpdiCl is present depend on the composition of the solvent and on the acidity of the solution. The rate of oxidative photodegradation of HpdiCl excitation in its Q bands (WBI) and the ability to produce free radicals are different for the different ionic species. The highest values correspond to the dicationic form of HpdiCl and the lowest values correspond to the neutral species. In the absence of oxygen, the formation of free radicals due to the reaction of 3(Hp dication) is detected in the following solvent mixtures: CH3OH-toluene, CH3OH-ethylbenzene, CH3OH-hexane. The data obtained indicate that interaction of 3(Hp dication) with methine groups is an intermediate step in the formation of free radicals. In the HpdiCl concentration range studied, the presence of a phenolic antioxidant, such as beta-naphtol, inhibits the oxidative photodegradation of the dicationic form in a treated solution, but has little effect on the oxidative photobleaching of the monocation. The rate of oxidative photodegradation of the monocationic form increases with the addition of propionic acid to the solution.