The first part of this paper concerns the thermal cracking of ethane in a pilot plant, under conditions representative for industrial operation, and the second part deals with the cracking of mixtures of ethane and propane. The rate of cracking of ethane is found to be decreased by the addition of propane, while the rate of propane cracking is slightly increased by the presence of ethane. Correlations are given for the rate coefficients of ethane and propane cracking in the mixtures. Global rate coefficients are also given. The product distribution was determined as a function of the reaction conditions and the mixture composition. The deviations from the pure additivity behavior are explained and correlated. A molecular reaction model was derived from the experiments. It permits a successful simulation of the cracking of ethane and mixtures of ethane-propane in industrial reactors.