Ethylene oxide was prepd. by the oxidn. of C2H4 by O over a Ag catalyst under non-stationary conditions, with the amt. of C2H4 and/or O being varied periodically. Thus, with C2H4 being varied from 0 to 12 mole % for 20 h at 10% C2H4 conversion, an increase in selectivity of 30.7% for ethylene oxide (compared to stationary conditions) was achieved. [on SciFinder (R)]