1 Introduction

The evolution of the physical parameters (Sari et al. 1998) of Gamma-Ray Burst (GRB) afterglows is of crucial importance in the knowledge of these phenomena, and can be analyzed by means of the radio-to-X-ray Spectral Energy Distribution (SED) observed at various epochs. By studying multiwavelength spectral variability and in particular the change of the frequency at which the SED maximum occurs, the powering mechanism of the afterglow, as well as its hydrodynamic evolution, may be clarified. To this aim, we have studied eight GRB afterglows which have been promptly pointed by the BeppoSAX NFI and for which both an X-ray and optical transient have been detected. Here we report only on four of these for which well sampled SEDs are available at least at two epochs.