Abstract: A process for producing a toner for developing electrostatic images is disclosed. A polymerizable monomer composition is first prepared and dispersed in an aqueous medium by the action of high shear force to produce particles. The polymerizable monomer is then polymerized in a container having a jacket. The liquid temperature inside the container is controlled by feeding a high-pressure vapor into the jacket while adjusting the degree of vacuum inside the jacket. The production process can achieve a superior temperature response, can keep the reaction system at a constant temperature with ease and can realize shortened production time and reduced production cost.