The aim of this work is to study the miscibility of poly(methyl acrylate)/poly(methyl methacrylate), (PMA/PMMA), sequential interpenetrating networks, (IPNs), as a function of the crosslink density using dielectric and dynamic-mechanical techniques. The PMA/PMMA system is immiscible and so, for low crosslink densities, phase separation appears, as detected by the occurrence of two clearly differentiated main dynamic-mechanical relaxation processes corresponding to the two components. If crosslink density is high enough, a homogeneous IPN can be obtained, achieving a forced compatibilization of both networks. The IPN crosslinked with 10% ethyleneglycol dimethacrylate shows a single main dynamic-mechanical relaxation process. Only the α main relaxation process appears in the PMA networks within the temperature range (−60 to 200°C) of the experiments conducted in this work. The dielectric relaxation spectrum of PMMA networks shows the secondary β relaxation followed by a small α relaxation partially overlapped with it. In the IPNs, both the main relaxation processes tend to merge into a single one and the dielectric spectrum shows a single peak that mainly corresponds to the secondary relaxation of the PMMA.