A continuous fractionation method for mixed particulate solids has been developed. When applied to suspensions of solids having substantially equal densities, the separation method distinguishes the particulates on the basis of their compressibility. The technique can be applied to solids ranging from micron to millimeter scale with overlapping or distinct particle size distributions. The method involves suspension of the solids into an appropriate processing medium, the application of a standing acoustic wave field to sort the particles across a separator channel, and the use of laminar flow fields along the channel to accomplish the ultimate separation. An analytical model of the separation process, based on particle trajectories through the separator chamber, is presented to show the sensitivity of the method to system properties acid to illustrate its selectivity. Results of prototypical experiments with polymer particles further demonstrate the method.