The thermal conductivity lambda and heat capacity per unit volume rho c(p) of poly(isobutylene)s, one 2.8 in weight average molecular weight and one 85 kg mol(-1) in viscosity average molecular weight (PIB-2800 and PIB-85000), have been measured in the temperature range 170-450 K at pressures up to 2 GPa using the transient hot-wire method. At 297 K and atmospheric pressure, lambda = 0.115 W m(-1) K-1 for PIB-2800 and lambda = 0.120 W m(-1) K-1 for PIB-85000. The bulk modulus BT has been measured in the temperature range 170-297 K up to 1 GPa. At atmospheric pressure, the room temperature bulk moduli B-T are 2.0 GPa for PIB-2800 and 2.5 GPa for PIB-85000 with dB(T)/dp = 10 for both. These data were used to calculate the volume dependence of lambda,g = -(partial derivative lambda/lambda / partial derivative V/V)(T).At room temperature and atmospheric pressure (liquid phase) we find g = 3.4 for PIB-2800 and g = 3.9 for PIB-85000, but g depends strongly on temperature for both molecular weights. The difference in g between the glassy state and liquid phase is small and just outside the inaccuracy of g of about 8%. The best predictions for g are given by the theoretical model of Horrocks and McLaughlin. We have found that PIE exhibits two relaxations, where one is associated with the glass transition. The value for dT(g)/dp at atmospheric pressure (for the main glass transition) is about 0.21 K MPa-1 for both molecular weights.