Background. The aim of this study was to investigate the effect of calcium and sucrose, which are in several cases present in fruit-based foods during food processing, on the thermal inactivation kinetics of pectin methylesterase extracted from tomato fruits. Material and methods. Pectin methylesterase from tomato fruits was extracted using 0.2 M KH2PO4/K2HPO4 buffer containing 1.0 M NaCl (pH 8.0) (1:1 w/v). The activity of pectin methylesterase was determined using an automatic pH-Stat. The thermal inactivation kinetics of the crude tomato pectin methylesterase extract were studied in the absence or presence of calcium (50 mM) or sucrose (20%) in a temperature range of 60°C to 69°C and calculated using non-linear regression analysis (using SAS 9.1 software). Results. The thermal inactivation of tomato pectin methylesterase followed a fractional-conversion kinetic model, indicating the presence of a first-order inactivating thermal sensitive PME fraction and the occurrence of a thermostable PME fraction. Tomato pectin methylesterase is more stable in thermal treatment compared to pectin methylesterase extracted from other fruits. Calcium and sucrose retarded the inactivation of pectin methylesterase but did not change the behavior of the enzyme during thermal treatments. Conclusion. Calcium and sucrose had protective effects on pectin methylesterase activity during thermal treatments. This finding is a good point of reference for food processors working in the area of fruit juice processing. Keywords: activation energy, fractional-conversion model, PME, protective effect, thermal inactivation, thermal stable fraction