Hybrids are characterized by the combination of organic and inorganic phases interacting through primary and/or secondary bonds [1–3]. These materials can be described in terms of particulate and interconnected or co-continuous morphologies. Particulate hybrids are usually produced by embedding inorganic phases in the form of discrete units in a polymeric matrix. Co-continuous hybrids, conversely, are characterized by interconnected nanoscopic domains and are usually generated from a precursor formerly introduced within the organic matrix [2–6]. The co-continuous morphology is generally the aimed one, since it creates the conditions to achieve the most efficient mechanism for the transfer of external excitations through the two phases, thereby maximizing the contribution of both components (organic and inorganic) to the overall properties of the hybrid (synergistic effect) [6–9]. This is particularly desirable in the case of foams, where the low density contributes to functional properties, such as thermal and acoustic insulation, while the inorganic reinforcement increases the stiffness and strength and dimensional stability, as well as water vapor permeability, improving also fire resistance and the adhesion to inorganic binders [10–12]. Recently, we have developed [10, 11] a new interconnected hybrid material produced by a non-conventional approach, which does not involve the usual sol–gel reactions [2, 6, 7]. The new hybrid is a porous material, in which the solid phase is composed of polyurethane and hydrated Portland cement particles. In this novel approach, the hybrid is produced in two steps: (a) formation of a polyurethane– anhydrous cement composite foam (see Scheme 1, composite material) and (b) formation of the hybrid through the hydration reaction of the Portland cement particles to produce the amorphous inorganic phases such as hydrated calcium silicates and aluminates that are commonly observed in the hydration of ordinary cement mixtures (see Scheme 1, hybrid material). This newly formed inorganic hydrated phase has the classical needle-shape structure of ordinary cements. The formation of an interconnected inorganic network depends on the original amount and dispersion of anhydrous cement particles within the polyurethane foam, as well as on the extent of the hydration [10, 11]. This new approach can be referred to as ‘‘Hydration-Induced Reinforcement of Polymer–Cement’’ (HIRP-C) hybrid materials. In view of the higher resistance to swelling of the inorganic phase when in contact with solvents, solvent sorption measurements have often been used to discern the presence of particulate or interconnected and co-continuous morphologies [13]. In this letter, we report the results of solvent swelling measurements to highlight the strong influence of the interconnected inorganic acicular structure type reinforcement within the walls of the cells of newly developed polyurethane/cement hybrid foam. In particular, we demonstrate that before the hydration step the cement particles are discretely dispersed in the polyurethane matrix, while, after the hydration step, a state of interconnected network is acquired, which provides a high resistance to solvent L. Verdolotti (&) M. Lavorgna S. Iannace Institute of Composite and Biomedical Materials, National Research Council, P.le Tecchio 80, 80125 Naples, Italy e-mail: lverdolo@unina.it