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Measurements have been made, at temperatures between 35 and 100° C, of the viscosities and thermal conductivities of gaseous methane, ethane, ethylene, propane, propylene,cyclo-propane,n-butane,cis-2-butene,trans-2-butene, 1:3-butadiene,iso-butane,n-pentane,iso-pentane,neo-pentane,cyclo-pentane,n-hexane,neo-hexane,cyclo-hexane, benzene,n-heptane, 2:4-dimethyl-pentane,n-octane, 2:3:4-trimethyl-pentane and of carbon tetrafluoride. Viscosities were measured by observing the damping of a pendulum swinging in the vapour and by a capillary-flow viscometer. Thermal conductivities were measured by the hot-wire method. The observed values of the ratio between the transport properties are discussed in the light of the kinetic theory of gases. I t is found that highly flexible molecules show systematically lower values of the ratio,k/n, than other molecules, and it is suggested th at this is due to a special type of intermolecular collision, involving some degree of intercoiling of the two molecules. This view is relevant to the interpretation of recent work on the vibrational activation of the ethylene molecule by collision with other hydrocarbons, and on the kinetics of the thermal decomposition of gaseous hydrocarbons. Inferences can also be drawn about the ease of interchange of translational energy with vibrational and with rotational energy in collisions; these are discussed in relation to information obtained from ultrasonic dispersion measurements.