Correlations between the photocatalytic activity and the chemical composition of carbon-supported P-Fe-based nanocomposites were found. Nanoporous carbon was prepared by chemical activation with H3PO4 of the endocarp of babassu coconut. The synthesis of the nanocomposites was conducted by aqueous impregnation of the support with Fe(NO3)(3)center dot 9H(2)O and NH4OH, followed by heat treatments under N-2 flow at temperatures from 700 to 1000 degrees C. The uptake of the composites to remove methylene blue by adsorption was studied previous to photodegradation tests under solar irradiation. The formation of iron phosphates and phosphides phases was accompanied by an enhancement of the adsorption capacities of the nanocomposites. The formation of iron phosphates (FePO4) enhanced the methylene blue degradation, with a remarkable increase in the photocatalytic activity up to 10 times higher than the commercial activated carbon. The increase of the temperature up to 900 degrees C led to a reduction of Fe(III) to Fe(II) pyrophosphates, with a concomitant decrease in the photoactivity. The formation of iron phosphides at 1000 degrees C was detrimental to the photocatalytic activity. It can be concluded that the P-Fe-based nanocomposites exhibited an important photocatalytic activity for the methylene blue degradation under solar irradiation, pointing to a promising application of these materials.