Pectolytic enzymes, viz., polymethylgalacturonase, polygalacturonase and pectinlyase were synthesised by Aspergillus niger NCIM 548 using submerged cultivation. The combined effect of pH and temperature on these enzymes was studied using response surface methodology. A central composite design for two variables was employed to study the combined effect of pH and temperature on stability. The optimum pH and temperature were found to be 2.23 and 23°C respectively for polymethylgalacturonase; 4.8 and 28°C respectively for polygalacturonase and 3.9 and 29°C respectively for pectinlyase. It has also been found that pectinlyase is more thermostable than other two enzymes. © 1998 Elsevier Science S.A. All rights reserved.