The monoglyceride selectivity obtained in the direct esterification of free fatty acids, such as dodecanoic acid or lauric acid, with glycerol is compared when catalysed either homogeneously or heterogeneously. Firstly, a relationship between the fatty acid chain length and temperature is established which quantifies auto-esterification. Subsequently, reaction conditions are chosen so as to avoid this homogenous reaction and to determine both the catalytic and selective effect of the zeolite. These conditions are valid until the critical micel concentration (CMC) of monoglycerides is reached. Due to the relatively low reaction temperature (385 K), side reactions other than esterification can be prevented. The use of solid catalysts limits the consecutive esterification into higher glycerides, thanks to a preferential adsorption of the polyol. Before emulsification of the reaction mixture, exclusively monoglycerides are formed on twelve-membered ring zeolites by shape selective esterification inside the pores. Variation of the crystal size shows that on ten-membered ring zeolites the reaction occurs on the external surface, consequently the monoglyceride selectivity is much lower and comparable to non-shape selective catalysts.