A theoretical and experimental study of mass transfer and mixing between a cold jet and a plasma stream is presented for an induction plasma reactor. The concentration profiles of a tracer gas (cold jet), injected axially in the main flow of the plasma, are measured along the centerline of the reactor. Through the application of a turbulent numerical model, concentration, temperature and flow fields are computed. By comparing the measured concentration profiles and the results of the modelling study, it is possible to deduce an effective mass diffusivity for the tracer gas in the mixture and thus calibrate the mathematical model. This work represents a first step in the study of diffusion and mass transfer phenomena under plasma conditions.