The thermal behaviour of pectin methylesterase (PME, EC. 3.1.1.11) in five new tomato cultivars used for juices and tomato products was analysed. Enzyme inactivation induced by heat was followed by measuring the activity as a function of temperature. The thermal behaviour of PME activity in five tomato varieties was described by sigmoidal thermal denaturation curves. The data analysis, performed assuming a two state equilibrium model, showed that only one of the five tomato varieties retained PME activity This observation could explain the cloud loss shown by some tomato products during storage. From the thermal transition curves, the D-T, Z and F/D parameters, usually employed for the evaluation of stabilization thermal processes, were calculated at various pH values.