The enhanced axial mixing which is caused by dispersion in oscillatory flows in some mass transfer devices may limit the reactor performance. This effect has provided the motivation for the present study in which oscillatory flow dispersion in a flat channel of large aspect ratio is investigated. The rate of spreading of a uniform slug of some passive tracer has been predicted using numerical and analytical techniques and the results have been verified experimentally. The numerical approach has used a finite difference time-marching method to obtain predictions for the channel concentrations. From the results, the dispersion coefficient (D) has been evaluated for Strouhal numbers of O.O1→0.2 and for mean Reynolds numbers of O.4→2OO at Schmidt numbers (Sc) O(1O³) . It has been concluded that under these conditions D varies as stroke squared. Unless the flow is not quasi-steady (i.e. if pulsatile Reynolds number α²