Blends of varying ratios of castor and soybean oils were used to produce biodiesel by alkaline transesterification using the methylic route. To increase the yield of esters, an experimental study using a full 2 4 factorial design was performed to evaluate the influence of methanol:oil molar ratio, KOH concentration, temperature, and reaction time on the biodiesel produced from each blend. Yields exceeding 95% were obtained, and the highest conversion was 99.05% at 25°C with a reaction time of 20 min using 2% KOH as a catalyst and a methanol:oil molar ratio of 12:1. To reduce process costs based on the amount of methanol used without limiting the conversion to esters, a second set of process conditions was identified, in which a 98.59% conversion to esters was obtained using the same temperature, reaction time, and catalyst concentration but a different methanol:oil molar ratio (6:1).