A simple inexpensive procedure for the control of the reactant transported by the carrier gas from solid sources is proposed. A provision for controlled redeposition of the reactant and modification of the carrier gas flow permit the variation of the mixing between the carrier gas and the reactant species. An approximate estimation of the pressure regime over which the reactant concentration varies significantly is made. The procedure has been experimentally verified in the case of Se vapor selenization of metal films. This procedure enables the use of total reactor pressure as a simple control parameter for a significant variation of the reactant availability which is very useful in the exploration of alternate reaction pathways.