b The sensitive pararosaniline-formaldehyde chromogenic system has been applied to the determination of sulfur as sulfur dioxide in blister and refined copper. Several factors affecting the combustion, furnace temperature, use of fluxes, and combustion accelerators and retarders have been investigated. Conditions established are quite different from those recommended by other researchers in the analysis of high melting metals and alloys. The effect of temperature on color development of the sulfonic acid derivative of prosaniline and its stability has been established for the first time. Interferences from common impurities present in copper such as selenium, tellurium, arsenic, and antimony in concentrations up to 10 times that of sulfur have been investigated. A procedure has been devised allowing accurate determination of sulfur in copper in the range 2 to 500 p.p.m. The standard deviation at the 10-p.p.m. level is less than i 1, and at the 100-p.p.m. level less than * 4.