The vapor pressures of isobutane (R600a) were measured from (259 to 406) K. The uncertainty of the vapor pressures was estimated to be less than 300 Pa. The measured vapor pressures were correlated using a Wagner-type vapor pressure equation. The critical pressure, normal boiling point and acentric factor were then determined from the vapor pressure equation. The speed of sound in gaseous isobutane was also measured along seven isotherms from (270 to 330)K using a cylindrical resonator. The perturbations from the thermal and viscous boundary layer, the gas fill duct, the shell motion and the vibrational relaxation were corrected in the resonance frequency measurements. The longitudinal acoustic modes were used to determine the speed of sound. The relative standard uncertainty in the speed of sound was estimated to be less than 0.015%. The ideal gas heat capacities and the second acoustic virial coefficients for isobutane were deduced from the speed of sound data. The second virial coefficients were obtained from the measured acoustic data and the square-well intermolecular model. (C) 2014 Elsevier B.V. All rights reserved.