The physical interpretation of the phenomenological rate constants of chemical kinetics is examined. For perfect gas systems, the traditional view of the rate constants for elementary reactions as one-way reaction fluxes is compared with a more recent interpretation (3) in terms of more complicated functions; it is concluded that, for real systems, the former view is a special case of the latter, while for systems in which the phenomenological rate equation is exact, the rate constants are shown to be one-way fluxes.In any given system, the applicability of the special interpretation is a function of the detailed nature of the transport matrix, which is generally unknown; practical implications of these ideas are presented. For systems other than perfect gases, the problem is complicated by the absence of well-defined molecular states; however, it is suggested that the flux interpretation is the most reasonable.