The sensitivity of neither the discretization time nor the mesh size is yet known in thermal modeling of geothermal systems. We determined their effect and optimal values in three-dimensional modeling of a magma chamber in two Mexican geothermal fields (Los Humeros, Puebla and La Primavera, Jalisco). Our results indicate that the discretization time is much more sensitive for the modeled subsurface temperatures than the mesh size. Therefore, all simulations must be carried out with the smallest discretization time (<= 10 years for geothermal fields), which would result in the largest number of time steps. The importance of the mesh size is better understood in terms of the resolution of neighboring wells in a geothermal area. Four 3-D runs were simulated in the La Primavera geothermal field, whose results generally agreed with the actually measured temperatures in geothermal wells. (C) 2013 Elsevier Ltd. All rights reserved.