Selectivity of product formation has been tested in hydrogenation of acetylene over 0.3 wt.% Pd/alpha-alumina and 0.5 wt.% Pd/TiO2 catalysts. Non-steady-state regime of catalyst operation was tested in pulse-flow experiments. Significant carbon poisoning appears to be a necessary requisite for selective formation of ethylene. The effect of hydrogen and acetylene partial pressure has been tested on the selectivity Of C-4 products. At 273-298 K the catalysts showed 26-35% selectivity for C-4 hydrocarbons and <2.5% for ethane production at conversions of 30-40%. Deuterium distribution in ethylene and 1,3-butadiene and the deuterium content of the surface hydrogen pool have been compared and mechanism of diene formation has been discussed.