The problem of transcritical mass transfer and combustion is considered in the counterflow geometry established at ambient pressures exceeding the critical pressure of the fluids but for an injection temperature (or injection temperatures) below critical. Real gas effects taking place under these conditions are treated with suitable models for thermodynamics and transport properties. A set of routines designated as ‘TransChem’ is constructed to extend the standard ‘Chemkin’ package to the transcritical regime. Three configurations are investigated. In the first case a low temperature oxygen stream impinges on a high temperature supercritical stream of the same substance and the ambient pressure exceeds the critical pressure. The structure of this flow is determined numerically and the mass transfer taking place between the dense oxygen and the hot stream is evaluated. It is shown that this rate can be correlated in terms of a transfer number which depends on the hot and cold temperatures and on the critical temperature. The mass transfer rate is also found to evolve like the square root of the strain rate. In the second case a flame is formed in the counterflow of a stream of cold oxygen injected at a temperature which is below critical and impinges on a supercritical stream of methane. In this flow, the flame is established in the light gas region adjacent to a sharp density gradient. The flame structure is close to that found in a gaseous situation except for the very large density change on the oxygen side which fixes the oxygen mass transfer rate to the flame. In the third case, the two streams are transcritical with temperatures below the respective critical values. The flow features two sharp gradients on the oxygen and methane sides and the flame is established between these two layers in the low density gaseous region.