The coherent optical response of high quality microcavities containing quantum wells is investigated. The dynamics of the ac Stark effect is investigated by degenerate pump-probe experiments, by exciting the quantum well excitons at various intensities and detunings. The evolution from a simple dynamical excitonic Stark shift to a splitting is dearly observed, even when the excitation is degenerate with the continuum states. The experimental results agree well with the predictions of a theoretical model based on the semiconductor Maxwell-Bloch equations.