Catalysts Au/Al2O3 based on gold nanoparticles exhibit high activity in the hydrogenation of phenylacetylene to styrene at 423 K. A decrease in the nanoparticle size from 8 to 2.5 nm leads to an increase in the turnover frequency of the reaction from 101 to 468 h(-1). A similar size dependence is observed for the activity of catalysts Pd/Al2O3. A decrease in the particle size of gold or palladium in monometallic catalysts, as well as the introduction of a second metal (Au, Zn, Ag) into the composition of palladium catalysts, leads to an increase in the rate of formation of styrene and a simultaneous decrease in the rates of formation of oligomers and ethylbenzene. The observed dependences are discussed in terms of the size and ensemble effects taking into account the mechanism of hydrogenation of acetylene derivatives.