Spectral　sensitization　of　a　mesoporous　graphite　carbon　nitride　(mpg-C(3)N(4))　photocatalyst　was　investigated　by　depositing　magnesium　phthalocyanine　(MgPc)　to　expand　the　absorption　to　wavelengths　longer　than　those　of　the　principal　mpg-C(3)N(4).　The　obtained　sample,　MgPc/Pt/mpg-C(3)N(4)　(Pt　as　a　cocatalyst)　showed　stable　photocatalytic　evolution　of　hydrogen　from　aqueous　solution　in　the　presence　of　sacrificial　reagents　(triethanolamine),　even　under　irradiation　at　wavelengths　longer　than　600　nm.　Increasing　the　amount　of　MgPc　led　to　ordered　MgPc　aggregation　on　the　photocatalyst　surfaces.　The　rate　of　photocatalytic　hydrogen　evolution　was　highest　on　a　sample　with　an　amount　of　MgPc　corresponding　to　a　monolayer　on　the　Pt/mpg-C(3)N(4)　photocatalyst　surface.　The　obtained　action　spectra　of　hydrogen　evolution　and　the　observation　that　the　amount　of　evolved　hydrogen　substantially　surpassed　the　amount　of　MgPc,　confirm　that　the　introduced　MgPc　functioned　as　a　photocatalytic　sensitizer.