Determination of relative proportions of quartz grains having different provenance-process histories is a major objective of Fourier grain shape analysis. Such "unmixing" is the key to tracing sediment dispersal pathways and evaluating the relative importance of potential sources. Unmixing is achieved using the Q-mode programs EXTENDED CABFAC-QMODEL on shape frequency distributions. The efficacy of these algorithms was tested using synthetic mixtures as well as an empirical data set. Results indicate that, when used optimally, relative proportions can be estimated with a precision of + or - 3 percent.