As part of a general effort to understand the etching of metals in both plasma and chemical systems the etching of platinum foils in an oxygen plasma generated in a flow‐type microwave system was studied. It was found that very rapid etching (∼ 6 A˚/s) took place even at low power inputs (200 W). The principal plasma parameters, including oxygen atom concentration, ion concentration, and electron temperature, were measured as a function of distance below the microwave coupler. These were correlated to the rate of foil etching, which decreased with increasing distance from the coupler. On the basis of these correlations a simple mechanistic model was formulated. Etching of platinum in an oxygen plasma jet results from the concomitant action of oxygen atoms and high energy electrons.