The purpose of this study was to investigate a series of alkali-doped metal oxide catalysts for their activity in the transesterification of sunflower oil and waste cooking oil with methanol. The metal oxides used as supports were Al2O3 and ZnO loaded with KHCO3 and KNO3, respectively, at various concentrations and calcined at different temperatures. These catalysts appeared to be promising candidates to replace homogeneous catalysts for biodiesel production as the reaction times and catalyst amount are low enough in order to achieve high ester yields. In most cases, the increase in loading concentration favorably influenced oil conversion. On the other hand, at calcination temperatures above 750 degrees C, a noticeable drop in biodiesel yield was observed which may be ascribed to decomposition effects of the active sites. In general, the use of sunflower oil resulted in higher ester yields when compared to waste cooking oil which was characterized by a high amount of free fatty acids and moisture. Biodiesel ester content was also strongly related with catalyst amount, methanol to oil molar ratio, reaction time, and catalyst reusability.