Crosslinked poly(methyl methacrylate) (PMMA-c), poly(carbonate-urethane) (PCU-c), poly(vinyl pyridine) (PVP-c), and full, simultaneous interpenetrating polymer networks (IPNs) based on the above polymers were characterized by precise heat capacity (C-p) measurements in the temperature interval 4.2-450 K. The raw values of C-p scaled with temperature (T) as C-p similar to T-d with d = 2 and 5/3, as expected for a fracton-like vibration regime, in the temperature intervals 8-10 and 10-30 K, respectively. A single glass transition temperature (T-g) and two T-g’s were observed for apparently homogeneous and microphase-separated IPNs, respectively. Judged by the positive sign of the excess Gibbs free energy, the apparently single-phase state of homogeneous IPNs is thermodynamically unstable; however, its kinetic stability is ensured by permanent topological constraints (network junctions) prohibiting the incipient phase separation.