Crystal size of epidotes crystallized in quartz + epidote veins is used as the basis for a new geothermometer from the fossil geothermal field of Saint Martin (Lesser Antilles). The epidote-bearing alteration paragenesis is developed as far as 3 km from a quartz diorite pluton at temperatures of 220-350 °C. The length/width ratio of the epidote grains is constant for all the analyzed samples and suggests "isotropic" growth environments. However, the length and width of the grains vary exponentially with temperature. The obtained results offer new perspectives for simple grain-size geothermomeery but must be extended to other geologic environments to clarify the influence of different rock types.