A method is described for stabilizing organic substrate materials having an absorption maximum in the range of from about 300 nm to 800 nm to light which comprises making at least one complex represented by the formula (I) coexist with the organic substrate materials in an amount effective to stabilize said organic substrate material: ##STR1## wherein M represents copper, cobalt, nickel, palladium or platinum, and R.sup.1 and R.sup.2 independently represent a hydrogen atom, an alkyl group, an aryl group, an acyl group, an N-alkylcarbamoyl group, an N-arylcarbamoyl group, or an N-alkylsulfamoyl group, an N-arylsulfamoyl group, an alkoxycarbonyl group or an aryloxycarbonyl group.