The catalytic performance of Pt-Cu/C with Pt/Cu atomic ratios of 1:1 and 1:3 in the reaction of 1,2-dichloroethane dechlorination at 200degreesC and atmospheric pressure was investigated to understand the molecular phenomena governing the change in ethylene selectivity with time on stream (TOS). When the Ptl-Cu3/C catalyst is prereduced in H-2 at 220degreesC, ethane is the major product at early TOS, while ethylene is the major product at longer TOS. Pretreatment in flowing He at 220degreesC eliminates the transient behavior and significantly increases steady state selectivity toward ethylene for both Pt-Cu catalysts. The transient behavior is also eliminated after reduction at 220degreesC followed by treatment with HCl at the same temperature. Quite different behavior is observed with a Pt/C catalyst in that ethane is the major product regardless of the type of pretreatment. The results are interpreted via a molecular scheme including the formation of mixed PtCuClx, species during the non-reductive pretreatment. These PtCuClx, particles are converted into the corresponding alloy during subsequent reduction by H2 or reaction mixture. The enrichment of the catalysts' surface with Cu results from the pretreatment with HCl.