Diacetyl is a byproduct of sugar manufacture and has many food-related uses. Vapor-liquid equilibrium (VLE) data are required in order to design a purification process. A vapor and liquid recirculating still was used to measure data for acetone + diacetyl at 303.15 K, 313.15 K, 323.15 K, and 40 kPa. Data were also measured for methanol + diacetyl at 313.15 K, 323.15 K, 333.15 K, and 40 kPa. The gamma-Phi approach to VLE was used to reduce the data to excess Gibbs energy model parameters. The vapor-phase nonideality was accounted for using the truncated (two-term) Virial equation of state. The Wilson, NRTL, and UNIQUAC excess Gibbs energy models were used to account for liquid-phase departure from ideal behavior. The data are shown to be reliable by both point and direct tests for thermodynamic consistency.