TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The bio-diesel product is obtained from the reactants by phase separation. The ionic liquid is recycled.Preferred Components: The ionic liquid is acidic or basic. The ionic liquid comprises a basic cation and neutral anion, neutral cation and basic anion, or both a basic cation and basic anion; and acidic cation and neutral anion, neutral cation and acidic anion, or both an acidic cation and acidic anion. The basic cation is a compound of formula (Cat+-Z'-Bas) (I) (preferably a compound of formula (N(Z'-Bas)(Rb)(Rc)(Rd))+ (Ia) or (P(Z'-Bas)(Rb)(Rc)(Rd))+ (Ib); especially 11 compounds are given in the specification e.g. a compound of formula (Ic) and (Id)). The basic anion is a compound of formula (Xb)-; and is selected from (F)-, (OH)-, (OR)-, (R-CO2)-, (PO4)3- or (SO4)2- (preferably (OH)-). The acid cation is a compound of formula ((T1)+-Z1-Acid) (II) (preferably 11 compounds are given in the specification e.g. a compound of formula (IIa) and (IIb); especially a compound of formula (IIc)). The acidic anion is a compound of formula (Xa)-; and is selected from (HSO4)-, (H2PO4)-, (HPO4)2-, (HCl2)- or (HX2)- (preferably (HF2)-, (HSO4)- and (H2PO4)-). The neutral cation comprises ammonium, phosphonium, pyrazolium, 1,8-diazabicyclo(5.4.0)-undec-7-ene (DBU) or 1,5-diazabicyclo(4.3.0)-non-5-ene (DBN); and is a compound of formula (N(Ra)(Rb)(Rc)(Rd))+ (IIIa) or (P(Ra)(Rb)(Rc)(Rd))+ (IIIb) (preferably 22 compounds are given in the specification e.g. a compound of formula (IVa) and (IVb)). The neutral anion is a sulfonate, phoshinate, triflamide (amide), triflate, dicyanamide, oxide (phenoxide) or halide anionic species, selected from (NTf2)-, (OTf)-, (R1-SO3)-, ((R1)2PO2)-, (Cl)-, (Br)- or (I)- (preferably (Me-SO3)-, (Ph-SO3)- or (Me-Ph-SO3)-).Cat+ = cationic species comprising ammonium, phosphonium, pyrazolium, DBU or DBN (preferably 1,3,5-trialkylpyrazolium, 1,2-dialkylpyrazolium or 1,2,3,5-tetraalkylpyrazolium);Z = covalent bond, or 1 - 3 aliphatic linking groups (where each linking group contains 1 - 10 carbon atoms and optionally 1 - 3 oxygen atoms) (preferably -(CH2-CH2)-, -(CH2-CH2-CH2)-, -(CH2-CH2-CH2-CH2)-, -(CH2-CH2-CH2-CH2-CH2)-, -(CH2-CH2-CH2-CH2-CH2-CH2)-, -(CH2-CH2-O-CH2-CH2)- or -(CH2-CH2-O-CH2-CH2-CH2)-);Bas = basic group (preferably N(CH3)2 or -N(CH(CH3)2)2);Rb - Rd = G1;G1 = H, 1-40C straight chain or branched alkyl, 3-8C cycloalkyl or 6-10C aryl (where the alkyl, cycloalkyl or aryl all optionally mono- - tri-substituted by 1-6C alkoxy, 6-10C aryl, CN, OH, NO2, 7-30C aralkyl or 7-30C alkaryl);R = 1-6C alkyl;T1 = cationic species (preferably heterocyclic ring selected from pyrazolium, isothiazolinium, tetrazolium, piperidinium, morpholinium or pyrrolidinium);Z1 = covalent bond or linking groups (where each linking group contains 1 - 10 carbon atoms and each optionally 1 - 3 oxygen atoms);Acid = acidic group (preferably -SO3H, -CO2H, -SO3-PH-R1, -SO3R1, R1PO(OH)2 or R2PO(OH));R1 = 1-6C alkyl or 1-6C aryl;R'b - R'd, and R1 - Rh = G1;any two of R'b - R'd, Re and Rf attached to adjacent carbon atom = -(CH2)q-;q = 8 - 20;X = F, Cl, Br or I.The plant fatty acid is derived from rape seed oil or prioline. The bio-diesel product and the ionic liquid are immiscible. EXAMPLE - Animal fat (lard -a triglyceride of mostly oleic acid) (1 g), methanol (2.09) and a catalyst of formula (H3C-N+(CH3)2-(CH2)2-OH) (OH)- (0.25 g) was placed in a microwave tube with a magnetic stirrer flea and heated at 100degreesC for 0.5 hour. This was cooled to room temperature and the two layers were analyzed to yield methyl ester of methyl oleate (bio-diesel) (yield 98%).