A compression/decompression (c/d) method was used to obtain experimental vapour-phase (p, rho, T) data for six mixtures of acetonitrile in n-butane for temperatures from 373.13 K to 433.11 K at 20 K intervals, and at pressures from 200 kPa to 1600 kPa. Density determination of the gas phase by the c/d method does not require knowledge of the mass of the compounds. The compression factors, the mixture second and third virial coefficients, and the interaction second and third virial coefficients of (CH3CN + n-C4H10) were determined. The compression factors are considered accurate to within +/-0.05 per cent, and the mixture second virial coefficients to +/-3 cm(3).mol(-1). Prediction methods for the estimation of cross second virial coefficients were also examined.