A method for determining sulfur trioxide (SO3) content of hydraulic cement is described. The method depends on heating the sample with the least possible amount of nitric acid. After filtration, the filtrate is diluted to a given volume and an aliquot passed through a cation exchange resin (hydrogen form) to separate all cations present. The eluant is diluted to volume, and an aliquot titrated against standard barium perchlorate solution using dimethylsulfonazo III (DMSA III) as visual indicator. Acetone helps detection of the equivalence point that is characterized by a color change from purple to sky blue. Seven Standard Reference Material portland cements and three commercially available cements were analyzed. Each cement sample was analyzed six times. The average absolute error for 60 determinations amounted to plus or minus 0.041%, and the pooled standard deviation for the 10 samples is 0.031%, for 50 degrees of freedom. All of the samples analyzed passed the t test at the 99% level. One sample determination consumes about 2 hr. The method requires no equipment other than a pH meter and ordinary glassware.