The National Institute of Environmental Health Sciences incinerates low-level radioactive waste containing 3H, 14C, and 35S as a means of volume reduction. The primary isotope remaining in the ash is 35S. Some effects of the chemical and physical form of the waste on sulfur retention in ash and several methods for determining the activity remaining in the ash have been explored. It was found that the sulfur in sodium sulfate has a higher retention in ash than the sulfur in methionine and that the presence of glass appears to enhance the retention. Liquid scintillation and proportional counting methods were compared. A sulfur extraction procedure using liquid scintillation counting was found to give the lowest minimum detectable activity. However, it was determined that a more practical method for routine monitoring is to suspend a small quantity of ash in a liquid scintillation gel.