New experimental data for the thermal conductivity of isobutane are reported that allow improved correlations to be developed. Previous correlations have been limited by a lack of thermal conductivity data for the vapor and compressed liquid at temperatures below 300 K and near the critical point. In addition, significant discrepancies were noted in the available high-temperature data for thermal conductivity. These new experimental data, covering the temperature range from the triple point at 113.55 K to 600 K and the pressure range 0.1 MPa to 70 MPa, are used together with the previously available data to develop improved correlations for the thermal conductivity of isobutane. The data reported here are estimated to have an uncertainty of +/-1% for measurements removed from the critical point and at pressures above I MPa, which increases to +/-3% in the critical region and +/-4% at low pressures (<1 MPa) at a 95% confidence level. The quality of the new data is such that the thermal-conductivity correlation for isobutane is estimated to have an uncertainty of about 3% at a 95% confidence level, with the exception of state points near the critical point and the dilute gas, where the uncertainty of the correlation increases to 5%.