A simple correlation, requiring the specific gravity as the only input, is found to predict the thermal conductivity of undefined petroleum and coal liquid fractions with comparable or better accuracy than other methods. The correlation was derived from 64 data points for 25 undefined petroleum fractions, and tested against 13 undefined petroleum fractions and 39 data points for 5 coal liquid fractions to give 3.930/a and 3.99% AAD, respectively.