The composition of the vapor phase over sulfuric acid has not been measured experimentally because of the low volatility of H{sub 2}SO{sub 4}. A method is described for calculating the partial pressures of H{sub 2}O, H{sub 2}SO{sub 4}, and SO{sub 3} based upon liquid-phase partial-molal thermodynamic quantities. Tables and graphs are provided which give the above partial pressures from -50 to 400 C at thirty-six compositions between 10 and 100 weight-percent acid.