Molar heat capacities at constant volume (C(V)) were measured with an adiabatic calorimeter for pure isobutane. The high purity of the samples was verified by chemical analysis. Temperatures ranged from the triple point of isobutane near 114 K to the upper temperature limit of the calorimeter at 345 K, whereas pressures ranged up to 35 MPa. Measurements were conducted on liquid in equilibrium with its vapor and on compressed liquid samples along isochores. Heat capacity results are reported for two-phase (C(V)((2))), saturated liquid (C(sigma)), and single-phase (C(V)) isochores. Vapor pressure data are reported that are based on measurements of C(V)((2)) along a two-phase isochore. Measurements were also made to determine the triple-point temperature of (113.707 +/- 0.030) K and enthalpy of fusion of (4494 +/- 20) J.mol(-1) for isobutane near its triple point. The principal sources of uncertainty are the temperature rise measurement and the change-of-volume work adjustment. The expanded uncertainty (i.e., a coverage factor k = 2 and thus a two-standard deviation estimate) for values of C(V)((2)) is estimated to be 0.5%, for C(sigma) it is 0.7 and for C(sigma) it is 0.7%.