A horizontal parallel plate apparatus was designed and constructed for the measurement of thermal conductivities of liquids. The liquid was held as a thin circular disk while the heat flux was downward through the liquid disk. The thickness of the liquid can be varied so the effect of thickness can be studied. The precision of the instrument is such that the results are within 1 per cent of the mean value. The thermal conductivities of nineteen liquid organic compounds were determined at 20 and 60 C. The compounds included isomeric ethers of varying structure, some esters of varying structure, and a hydrocarbon. All theoretical equations and empirical correlations for the prediction of the thermal conductivities of liquids were tested using the thermal conductivities of the nineteen liquids. The only available physical characteristic which would correlate the experimental results was the ASTM viscosity slope, and compounds containing rings did not correlate in this case. The experimental results were found to correlate best with the number of atoms in the longest chain. Corrections can be made for the effects of side chains and rings.