The effects of gallia addition on the Pt dispersion and the activity for ethylene hydrogenation at 0°C were studied for Pt-supported catalysts as a function of the reduction temperature (350, 450, and 550°C). The catalysts contained 0.5 wt.% Pt and were prepared by successive incipient wetness impregnations with Ga(NO 3 ) 3 and H 2 PtCl 6 aqueous solutions. CO and H 2 chemisorption data indicated that, the addition of small amount of Ga 2 O 3 caused an increase of the Pt dispersion and a decrease of ethylene conversion. Both of them decreased appreciably when Ga 2 O 3 addition was increased, particularly, for the case where bGa 2 O 3 was used as a support. The increase in reduction temperature magnified the negative effects of the gallia addition on dispersion and activity, although the addition of small amount of gallia improved the resistance to metal sintering. Results were interpreted in terms of the presence of reduced Ga species, which can encapsulate Pt particles.