The platinum nanocomposites were synthesized by chemical reduction of H2PtCl6·6H2O in situ with a methanol—water mixture using mesoporous graphitic carbon nitride as a stabilizing matrix and a catalyst support. The textural, morphological, and optical properties of the obtained platinum – mesoporous carbon nitride composites (Pt/mpg-C3N4) were studied. Pt/mpg-C3N4 has been developed as an effective heterogeneous catalyst for the gas and liquid phase hydrogenation of phenylacetylene. An efficient and versatile approach to the modification of platinum with various organic solvents and ligands for the selective hydrogenation of phenylacetylene to styrene in a flow gas-phase process is pre-sented. The addition of pyridine, piperidine, or morpholine to the starting solution of phenylacetylene in hexane provides 95–100% conversion and 90–92% selectivity to styrene. A sharp increase in the selectivity to styrene without substantial changes in conversion is also observed when THF, ethanol, triethylamine, dioxane, or chloroform are used as diluents. The obtained Pt/mpg-C3N4 composites also demonstrated considerable catalytic activity in the selective hydrogenation of phenylacetylene to styrene at low temperatures in the liquid phase