H-1 NMR spectroscopy revealed the formation of a strong interaction between sulfortated poly(ether ether ketone) (PEEK) and the casting solvents dimethylformamide (DMF) and dimethylacetamide (DMAc), commonly used in proton exchange membranes (PEM) preparation. The DMF amide function readily forms a hydrogen-bonding complex with sulfonic acid group starting at temperatures as low as 60 degreesC. DMAc is less susceptible to participate in this reaction and hydrogen-bonding occurs in this case only above 100 degreesC. Both solvents are prone to thermally activated decomposition possibly accelerated by sulfonic acid functions and the produced dimethylamine (DMA) also forms a strong association with SPEEK. The solvents' decomposition is strongly catalyzed by residual sulfuric acid originating from SPEEK preparation, with the resulting formation of N,N-dimethylaminium hydrogensulfate (DMAm). Both DMF and DMAc solvent interaction with SPEEK sulfonic acid Groups significantly affect the properties of PEMs, lowering their proton conductivity. (C) 2003 Elsevier B.V. All rights reserved.