The accurate measurement of growth and etch rates during plasma processing is difficult, if not impossible, to achieve for opaque, three-dimensional substrates. Sensing small weight changes that correspond to discontinuous film formation on a substrate is also difficult with traditional optical methods. Using a gravimetric recording microbalance, we have successfully measured small weight changes of aluminum samples during the pre-cleaning and nitriding stages in low temperature plasma. The data will show that the microbalance can provide useful information about the rates of various processes within a plasma. The data can be interpreted using reaction-diffusion equations to determine the relevant rate parameters for a plasma process. This approach can be extended to the study of many materials including oxides, metals, and nitrides subjected to various processes including etching and deposition.