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Textural changes during fruit ripening have been attributed to pectin degradation due to pectic enzymes such as pectinmethylesterase (PME, EC 3.1.1.11). PME catalyzes the de-esterification of pectin, a complex mixture of polysaccharides, namely methyl esterified polygalacturonic acid, with the release of hydrogen and methanol, producing shorter chains causing drastic losses in firmness. Thus, control of PME activity in fruits aiming at texture maintenance and/or improvement is extremely important to the food industry. However, PME activity and properties are dependent on product, environmental and physico-chemical conditions, such as pH and temperature. Therefore, the aim of the present work was to optimize the pectinmethylesterase (PME) enzyme assay from tomato (Solanum lycopersicum 'Zinac') fruits, and determine its kinetics behavior and thermal stability. The highest PME activity was found with a 1.0 M of NaCl extraction solution, with 0.5% citrus pectin, and revealed optima of temperature at 60°C and pH of 8.8. The low kM value (0.011%) for tomato PME describes the high affinity between enzyme and substrate (citrus pectin), whereas the obtained Vmax value (0.712 U mg-1) relates to the enzyme quantity present in the reaction. The study of PME thermal stability showed two distinct behaviors: an increase of activity from 40 to 50°C and a decrease from 55 to 80°C. At 50°C/10 min an increase in activity up to 17% of activity was observed. At 60°C, about 50% of the activity still remained after heating for 25 min, and PME was completely inactivated at 80°C after 10 min. Data obtained in the temperature range of 55 to 80°C were also satisfactorily described by an Arrhenius first-order kinetic model. These results provide useful information about the different factors that affect tomato PME activity and may be used as a tool for firmness control during postharvest handling and fruit processing.