Heel formation during cyclic adsorption/regeneration of volatile organic compounds (VOCs) on activated carbon decreases its adsorption capacity and lifetime. The effect of regeneration purge gas oxygen content on activated carbon performance, specifically during successive adsorption/regeneration cycles was investigated. 5 cycle adsorption/regeneration tests were performed on a microporous activated carbon using 1,2,4-trimethylbenzene (TMB) as adsorbate. Nitrogen with different oxygen concentrations (<= 5, 208, 625, 1250, 2500, 5000, 10,000, and 20,000 ppm(v)) was used as regeneration purge gas during thermal desorption of TMB (at 288 degrees C). Cumulative heel formation increased from 0.5% to 15.8% as the oxygen concentration in the desorption purge gas increased from <= 5 to 20,000 ppm(v). Thermogravimetric analysis of the regenerated samples showed extensive chemisorption of TMB when exposed to a >= 625 ppm(v) oxygen in the purge gas. The results suggest that regeneration purge gas oxygen impurity can increase heel formation, resulting in lower regeneration efficiency and shorter adsorbent lifetime. The results from this study help explain the heel formation mechanism and how it is related to regeneration purge gas purity.