Using reliable data on thermal conductivitiesK, viscositiesη, and specific heatscv, values of the important ratiof = K/ηcvhave been determined over a wide range of temperature, 80–380°K, for eight gases. For some gases the ratio shows a decrease with increasing temperature and for others an increase. The results and existing theories do not agree on all points. For a monatomic gas, helium,fhas a constant value up to 250°K, agreeing with most theories, but it drops at higher temperatures. One diatomic gas, hydrogen, shows a sharp drop, agreeing very well with Eucken's theory. Another diatomic gas, nitric oxide, shows a rise much sharper than predicted by the theory. Other diatomic and polyatomic gases show large deviations from Eucken's theory, mainly in that experimental values rise while the theory predicts a decrease with increasing temperature.