Invertase was immobilized into polyacrylamide-gelatin carrier system by chemical cross-linking with chromium (III) acetate, chromium (III) sulphate, and potassium chromium (III) sulphate. The optimum conditions, namely substrate concentration, temperature, and pH were determined. The effect of polyacrylamide-gelatin ratio and cross-linker concentration oil immobilized enzyme activity were analysed. Maximum immobilized enzyme activities were obtained with chromium (III) acetate (0.01 mol dm(-3)), chromium (III) sulphate (0.004 mol dm(-3)) and potassium chromium (III) sulphate (0.001 mol dm(-3)) for 0.177 (w/w) polyacrylamide-gelatin carrier ratio as 79%, 72% and 79%, respectively. The K-m values were 86 and 166 mM for free and immobilized enzyme, respectively. All immobilized samples were used 20 times over a period of 2 months without a considerable loss of activity. (c) 2005 Elsevier Ltd. All rights reserved.