Segmental relaxations in a series of poly(propylene oxide)-based polyurethane/butyl methacrylate-triethylene glycol dimethacrylate copolymer interpenetrating networks (IPNs) of various compositions, as compared to those in the pure constituent networks, were studied by an original laser-interferometric Creep Rate Spectroscopy (CRS) technique. The spectra, obtained over the range from 150 K to 360 K, confirmed the CRS superiority in resolution to generally utilized relaxation spectrometry techniques and allowed to characterize in detail the heterogeneity of segmental dynamics within or near the extraordinarily broad glass transition range in these IPNs. Up to eight creep rate peaks have been registered which were shown to be associated just with the predicted kinds of segmental motions, cooperative and partly- or noncooperative; thereby, molecular assignments could be done for these peaks. The relative peak contributions to dynamics around T-g as a function of IPN composition were approximately estimated that provided also some information on nanoscale compositional inhomogeneity of the networks.