Small-angle neutron scattering (SANS) was used to study the effects of cross-link density on the structure of in situ sequential interpenetrating polymer networks (IPNs). The system investigated is a PSD (deuterated polystyrene-co-divinylbenzene) network built within a matrix network of PUR (polyurethane). The scattering can be approximated by the Debye-Bueche law with a q(-4) dependence at intermediate values of wave vector q, with departures from it at both low and very large values of q. At low q, this Debye-Bueche law yields a value of the structure factor S(q-->0) and a correlation length xi. At large q, it yields a value of the specific area S/V S being the total area and V the total volume. As the cross-link density of either network of the IPN increases, S(q-->0) and xi decreases, while S/V increases. The measured xi is in the range 20 to 80 Angstrom and the values of SIV correspond to 100-300 m(2)/gm, indicating a rather finely divided material. The quantity S(q-->0), after being divided by volume xi(3), allows us to estimate the contrast. The existence of a maximum in S(q) is discussed for the most cross-linked samples. With increasing cross-link density of the networks, it seems that the size of phase-separated regions is decreasing, indicating that the IPN structure freezes earlier in the evolution of the separation kinetics. However, from comparison of effective contrast and maximum contrast between "pure phases" (i.e. made of pure PSD and the other of pure PUR), it seems that the content of the separated regions is closer to "pure phases" when the cross-linking is higher. At this stage of the analysis, the effects of complicated topological connectivity between the two networks were neglected.