[en] There are two definitions of the concentration of dissolved or suspended material in a flowing fluid, namely the resident concentration and the flux concentration. The validity of the dispersion equation with a chemical reaction term is considered for the cases of the flux concentration. This concentration has to be considered when the mass transfer is of interest. Depending on the injection-detection mode the solute is either injected and measured in terms of the flux concentration or in terms of the resident concentration. It is shown which solutions to the dispersion equation are most adequate to the common experimental situations. Problems related to the determination of the mean flow velocity and the dispersion coefficient from tracer experiments are considered. A new simple method of interpretation suitable for highly dispersive systems is proposed. (author)