A study was performed of the transesterification reaction of used frying oil by means of methanol, using sodium hydroxide, potassium hydroxide, sodium methoxide, and potassium methoxide as catalysts. The objective of the work was to characterize the methyl esters for use as biodiesels in compression ignition motors. The operation variables used were methanol/oil molar ratio (3:1-9:1), catalyst concentration (0.1-1.5 wt %), temperature (25-65 degrees C), and catalyst type. Also, experiments in two stages of reaction, with separation of the glycerol in the first stage, were carried out. The evolution of the process was followed by gas chromatography, determining the concentration of the methyl esters at different reaction times. The biodiesel was characterized by its density, viscosity, high heating value, cetane index, cloud and pour points, characteristics of distillation, flash and combustion points, saponification value, and iodine value according to ISO norms. The biodiesel with the best properties was obtained using a methanol/oil molar ratio of 6:1, potassium hydroxide as catalyst (1%), and 65 degrees C temperature. This biodiesel had properties very similar to those of no. 2 diesel. The two-stage transesterification was better than the one-stage process.