A numerical model has been developed to describe the characteristics of the recently developed micro open parallel plate separator (muOPPS). This model solves the mass-balance equation for a single solute within a microchannel with a rectangular flow cross-section, and accounts for the velocity profile in the channel. It is shown that the separation characteristics are sensitive to the concentration and velocity gradients along the depth-a factor that has been neglected in the past. An empirical expression has been developed for the reduced plate height of the muOPPS. It has been shown that estimation of performance parameters using an equivalent radius in equations developed for a circular cross-section gives erroneous predictions.