We derive and analyze the relaxation frequency v and the shift Deltaomega of excitonic resonances in the regime of weak confinement (also known as thin film regime). In calculating v and Deltaomega, we have solved the integral equation for the averaged Green's function within the self-consistent Born approximation. The dependencies of v and Deltaomega on the parameters of the excitonic thin film and its rough interfaces are found. We present specific results for CuCl, where the thin film regime is maintained up to a very small sample thickness. (C) 2003 Elsevier Science B.V. All fights reserved.