This is a progress report on modelling the first of two aspects of plasma assisted chemical processes; it is shown here that by maintaining a constant plasma geometry, observable plasma parameters can be made to determine ion assisted surface chemistry uniquely because the mean energy of ions bombarding the reacting surfaces are thus also uniquely determined. Furthermore, the theory (1) given here sets the stage for another, given elsewhere (2), whicn shows that the resulting processes (i.e., reactive ion or plasma assisted etching and deposition) follow the same “rules”, as thermally driven cnei.lical vapor transport (CVT) for a fixed mean ion energy. Together, these theories permit a considerably more orderly understanding and application of plasma etch/deposition processes than has oeen previously possible.