TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: Membrane separation is carried out at the end of a first reaction stage carried out in a first conversion reactor (2), on a stream (E) containing mainly FAE, MHA, GL, partially converted glycerides and traces of water (obtained after GL removal); and provides a retentate (E1) and permeate (E2). The separation is carried out in a separation unit (12) with at least one membrane, at -30 to +150 degrees C (preferably room temperature to 90 degrees C). The pressure difference between the faces of the membrane(s) is 1-15 (preferably 2-8) MPa. The membrane has a degree of separation of at least 50% and a degree of GL retention of more than 30% (preferably more than 50%). Membrane separation is followed by a second reaction stage in a second conversion reactor (7). The retentate (E1) (containing most of the unconverted tri- and di-glycerides) is recycled to the first reactor and the permeate (E2) is supplied to the second reactor; alternatively the retentate is supplied to the second reactor and the permeate is mixed with the effluent from the second reactor before being supplied to a purification section with an alcohol evaporator (8), insoluble GL separator (10) and/or final purifier (11). In a further alternative, if the permeate meets required glyceride content specifications it is supplied directly to the purification section (i.e. no second reactor is used). EXAMPLE - Transesterification of oil with methanol was carried out as shown in the figure, according to the basic procediure described in EP1352893 but additionally using a membrane separation unit (12) in which the membrane had a selective layer of based on thermally post-treated polyimide. The performance profile of the membrane in fractionation of glycerides was degree of extraction 75%, degrees of rejection of tri-, di- and mono-glycerides 73%, 64% and 40% respectively and degree of rejection of esters 0%. The flow rate of the starting oil (A) (100% triglycerides) was 10 t/h. The flow rate of methanol (B) introduced before the first reactor (2) was 5.4 t/h. The ester-rich upper phase (E) from the decanter (6) had a combined glyceride and ester flow rate (GEFL) of 13.8 t/h and a methanol flow rate of 0.2 t/h; and contained (by weight) tri-, di- and mono-glycerides at 8.1%, 2.8% and 83.1% respectively and ester at 83.1%. The retentate (E1) from the membrane separator had GEFL 3.5 t/h; and contained tri-, di- and mono-glycerides at 25.9%, 8.2% and 8.3% respectively and ester at 57.7%. The permeate (E2) from the membrane separator had GEFL 10.3 t/h; and contained tri-, di- and mono-glycerides at 2.2%, 1.0% and 2.3% respectively and ester at 91.6%. The flow rate of alcohol (G) introduced before the second reactor (7) was 5.2 g/h. The final purified ester (L) had GEFL 10 t/h; and contained tri-, di- and mono-glycerides at 0.2%, 0.2% and 2.8% respectively and ester at 98.8%. The cooling and heating energy requirements were 0.746 and 0.826 kW per hour respectively.