New experimental data on the thermal conductivity of methylcyclohexane and propylcyclohexane are reported that allow the development of wide-range correlations. These new experimental data, covering a temperature range of (300 to 600) K and a pressure range of (0.1 to 60) MPa, are used to develop correlations for the thermal conductivity. The experimental data reported here have an uncertainty of less than 1% for measurements removed from the critical point and for gas at pressures above I MPa, increasing to 3% in the critical region and for gas at low pressures (less than I MPa). On the basis of the uncertainty of and comparisons with the present data, the thermal conductivity correlations for methylcyclohexane and propylcyclohexane are estimated to have a relative uncertainty of about 4% at a 95% confidence level. The present data include a contribution due to thermal radiation that increases in proportion to temperature cubed up to an estimated value from 2% to 3% near 600 K.