Abstract: Process for obtaining cellular materials, based on unsaturated polyester in optionally containing reinforcing and/or inert fillers, having a density between 0.2 and 1.0 kg/l by moulding under pressure a liquid foam of said resins, characterized by the fact that the liquid foam based on unsaturated polyester resins containing conventional additives, optionally containing an inert and/or reinforcing charge of mineral or organic fibers having a length which is less than 5 mm, preferably less than 3 mm and more preferably less than 1.5 mm, obtained by mechanical introduction of a gas and/or by means of a chemical and/or physical foaming agent, is firstly introduced into an open half-mould optionally containing a further fiber reinforcement, by casting an amount higher than the free volume of the mould, and subsequently said foam is cross-linked.