An approximate solution for the transient Lévêque problem has been developed based on the approximate integral method and the method of weighted residuals. This solution can be used to explore the dynamic response of thermal or mass boundary layers in large amplitude, but non-reversing, unsteady flows. The accuracy of this approximate solution has been proven over a wide range of dimensionless frequencies ( ω = Ω(L 2/S 2 0D) 1,3) . The approximate solution forms the basis of a simple method for calculating the instantaneous shear rate from the measurement of instantaneous heat or mass flux at the surface of a wall transfer probe when the quasi-steady state assumption is not valid.