This work investigates two new aspects of the biphasic esterification reaction of glycerol (G) and oleic acid (A), i.e. the formation of several oxidation/dehydration co-products and the effect of the ester products as phase transfer agents to accelerate the reaction. Potentiometric titration, FTIR, TG and especially paper electrospray mass spectrometry (PS-MS) were employed to analyze the products. Reactions with different G:A molar ratios (1:0.5, 1:1 and 1:2) without catalyst at 140, 160 and 180 degrees C showed oleic acid conversions of 50-80% to form the mono, di and triesters with several other by co-products due to dehydrogenation/oxidation of the oleic chain and dehydration of the glycerol moiety identified by PS-MS. The phase transfer autocatalytic effect was investigated by the addition of the oleate esters (2, 4 and 10 wt%) to the reaction. It was observed a strong acceleration of at the initial stage of the process indicating an autocatalysis likely due to a phase transfer effect. The reaction is discussed in terms of three stages. In the initial biphasic Stage 1, oleic acid suffers some dehydrogenation/oxidation and the esterification reaction takes place slowly at the glycerol-oleic acid interface to form mainly the monooleate ester which is an amphiphilic/tensoactive molecule. In the Stage 2, the amphiphilic ester products act as phase transfer agents taking glycerol molecules to the oleic acid phase for the formation of mono and mainly dioleate ester. At the Stage 3 in the presence of relatively high concentration of the tensoactive ester products the reaction becomes homogeneous/monophasic.