ABSTRACT Experiments have shown that environmental oxygen has a very strong effect on track etching in CR-39. Post-irradiation treatment with oxygen affects both the track etch rate and the bulk etch rate. The effect on highly ionizing tracks appears to be to increase the preferential track etching, in this way reducing the cone of the track to a small value. The technique should be useful in a variety of applications involving the use of CR-39 where very thin channels are required.