An ethylene stream which contains ethane as an impurity or a propylene stream which contains propane as an impurity is subjected to adsorption at a temperature of 50.degree. to 200.degree. C. in a bed of adsorbent which selectively adsorbs ethylene or propylene, thereby adsorbing substantially all of the ethylene or propylene. The purified ethylene or propylene stream is then subjected to partial oxidation in the presence of oxygen and, optionally ammonia to produce various partial oxidation products. The process is operated on a low per pass conversion with recycle of unreacted ethylene or propylene. In the system of the invention the adsorption unit may be upstream or downstream of the partial oxidation reactor.