Rates of oxidation in oxygen at 140°C. of natural rubber gum vulcanizates containing PBNA as inhibitor have been compared. They are a function of sulfur combined with the rubber, and only slightly affected by curing system for sulfur alone, TMTD sulfurless, and very high-efficiency sulfenamide plus dithiocarbamate-accelerated compositions. Addition of diphenylguanidine to simple sulfur vulcanizates causes much faster oxidation. No conclusions about oxidation mechanism can be drawn from rates of oxidation alone.