Tetra(-4-pyridyl)porphyrinatomanganese(III)sulfate, MnP, (in the forms of MnIII and MnII mixture), was embedded into a polysiloxane polymer matrix and attached to the surfaces of n-GaAs wafers. The n-GaAs=polymer=MnP system was annealed under nitrogen and used for photoelectrochemical study in water=LiClO4=Fe(CN)3 6 = Fe(CN)4 6 system. The results indicated a positive shift in the value of the flat-band potential of the semiconductor due to MnP. This was manifested by shifting the values of the dark-current onset potential and the photo-current open-circuit potential towards more positive values. These findings are potentially valuable in future applications of solar energy in hydrogen and oxygen production from water.