The time evolution of the complex dielectric function of CuCl is studied under high excitation conditions at 4 K, through a “pump and probe” method with a 30 ps time resolution. The transmission of the probe pulse, optically delayed with respect to the pump pulse, gives information on the temporal dependence, on the respective density and energy distribution of the exciton and biexciton populations and of the resulting non-linear absorption. In reflection measurements on thin epitaxial layers of CuCl on CaF 2, this population dynamics gives to a time dependent shift of interference fringes and to a global attenuation of the exciton reflection.