The binary scaling law is commonly used to study the aerothermodynamics of hypersonic vehicles in ground-based facilities. Its application is usually restricted to situations where diffusive transport is neglected. However, as demonstrated in this paper, the diffusion processes are embedded in the Peclet and wall Damkohler numbers, which are conserved through the duplication of the binary scaling similitude parameters, namely the nature of the gas, the free-stream enthalpy h(infinity). and the product of a density and a length-scale of the flow rho L. The role of diffusion is first demonstrated theoretically, and then experimentally through the obtention of binary scaled boundary layers in the Plasmatron, a plasma wind tunnel at the von Karman Institute. The stagnation point heat fluxes were measured and exhibit a good agreement with the theoretical scaling law. This also validates the use of the binary scaling law in subsonic high enthalpy flows. (C) 2016 Elsevier Ltd. All rights reserved.