The cross second virial coefficients and dilute gas shear viscosities, thermal conductivities, and binary diffusion coefficients of the systems (CH₄ + C₂H₆) and (N₂ + C₂H₆) were determined at temperatures from 90 K to 1200 K using statistical thermodynamics and the kinetic theory of molecular gases. The required intermolecular potential energy surfaces (PESs) forCH₄–C₂H₆ and N₂–C₂H₆ interactions are presented in this work, while the like-species interactions were modeled using PESs from our previous studies on the pure gases. All of these PESs are based on high-level quantum-chemical ab initio calculations and were fine-tuned to the most accurate experimental data available for the second virial and cross second virial coefficients. The agreement of the calculated values for all investigated thermophysical properties with the best experimental data is overall very satisfactory and confirms the high accuracy of the calculated values.