Urease enzyme was immobilized in copolymer matrices containing poly(ethylene glycol) (PEG; MW 14000 and 1000) or polypropylene glycol (PPG; MW 2000), prepared with 2-hydroxyethyl methacrylate (HEMA) and/or N-vinyl-2-pyrrolidone (VP). Urease-immobilized copolymer matrices were stored in phosphate buffer solutions of pH 7.0 at 4 degrees C, and the activity of the enzyme was determined periodically for up to 90 days. The highest activity was observed for HEMA and VP containing PEG (MW 14000) matrix. The matrices comprising VP and PEG (MW 14000) did not have mechanical stability and degraded in 3 weeks, while all the others were stable and demonstrated enzyme activities for up to 3 months. The effective diffusion coefficients of urea in these polymer matrices were calculated with a ’diffusion and reaction model’ and the highest effective diffusion coefficient was found in the HEMA and VP containing PEG matrix, possibly due to its highly porous structure.