One of the problems in the understanding and implementation of the plasma etching process is the general lack of quantitative information about the effect of the many process parameters involved. We have been studying the effect of gas flow rates in the etching of silicon materials in fluorocarbon gases, and have been led to define a utilization factor to describe the extent to which the reaction gas is converted to volatile products of the plasma etching process. The experimental evidence for the importance of a utilization factor in plasma etching is shown, and some of the surprisingly many ways in which flow rates influence the whole etching process are illustrated.