Several variants of thermally stimulated currents (TSCs) have been used to characterize an unusual room-temperature (RT) Debye-type relaxation mode (denoted beta') recorded in the TSC spectra of radically polymerized atactic poly(methyl methacrylate) with a high syndio-triad content. The beta' signal is sensitive to variations in the adsorbed water content, the time of isothermal aging, and probably the presence of nonpolymerized monomers, unreacted initiator molecules, and radicals. Its molecular origin is extensively discussed under the assumption of the dipole reorientation of carboxymethyl side groups presenting hydrogen-bond interactions with adsorbed water molecules. An alternative explanation involves the relaxation of polar side groups that experience rotational hindrance through van der Waals and electrostatic interactions by ionic or molecular impurities. A preliminary analysis of the low-frequency epsilon"(f) spectrum (recorded at RT) suggests that the dielectric relaxation spectroscopy manifestation of the TSC beta' signal, when present, may possibly be confused with structural relaxation losses induced by high-temperature thermal treatments. (C) 2004 Wiley Periodicals, Inc.