Proton-conducting polymer electrolytes based on methacrylates were prepared by direct, radical polymerization of ethyl (EMA), 2-ethoxyethyl (EOEMA), and 2-hydroxyethyl methacrylate (HEMA). Samples with embedded solutions of phosphoric acid in propylene carbonate (PC), γ-butyrolactone (GBL), N,N-dimethylformamide (DMF) and their mixtures were studied using impedance, voltammetrical and thermogravimetric methods. Membranes of long-term stability exhibit ionic conductivity up to 6.7×10−5 Scm−1 at 25°C reached for the sample PEMA–PC–H3PO4 (31:42:27mol.%). The accessible electrochemical potential window is 2.2–3V depending on the working electrode material (glassy carbon or platinum). The thermogravimetric analysis shows that the membranes are thermally stable up to 110–130°C.