The density of the binary system 1-methyl-4-(1-methylethenyl)-cyclohexene (limonene) (1S,5S)-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (a-pinene) has been measured for nine different compositions and the pure components at five pressures from (20.0 to 40.0) MPa and six temperatures from (283.15 to 358.15) K with a vibrating-tube densirneter. The experimental uncertainty is estimated to be +/- 0.5 kg.m(-3). The isothermal compressibility and isobaric thermal expansion were derived from the experimental density data. Two different equations of state, conventional SAFT and PC-SAFT, were applied to predict the densities of the mixture. The best predictions were achieved with PC-SAFT.