NOVELTY - Production of fatty acid ester biofuels by (trans)esterification such that the procedure can be repeated with decreased amounts of added alcohol, alkyl aromatics and/or sulfuric acid is effected by pressureless reaction of a mixture of free fatty acids or a fatty acid- and triglyceride-containing material, followed by phase separation.    USE - In biofuel production.    ADVANTAGE - The catalyst is produced in situ from inexpensive components while still giving effective control of the reaction and easy work-up of the product. Setting of the sulfuric acid concentration allows the dewatering of the product and also the catalyst formation speed to be controlled and unreacted alkylaromatics can serve as entraining agents during product dewatering and can be recycled. The ester phase contains reduced amounts of catalyst residue and also of parasitic by-products. Further, the sulfuric acid can be recycled.    DETAILED DESCRIPTION - Production of fatty acid ester biofuels by transesterification and esterification such that the procedure can be repeated with decreased amounts of added alcohol, alkyl aromatics and/or sulfuric acid comprises:    (1) pressureless reaction in a tubular vessel of a mixture of free fatty acids or a fatty acid- and triglyceride-containing material, an alcohol of 3-5C chain-length and a mono- or bi-nuclear aromatic with greater than or equal to1 CH3 group in presence of 30-85 % sulfuric acid at 50-80 degrees C at a ratio of 1 : 2-2 : 1 in relation to the stoichiometry of the sulfonic acid production; and    (2) separation in a sedimentation tank into a heavy aqueous phase and an organic upper phase.    DESCRIPTION OF DRAWING(S) - The drawing shows the process without regard to monitoring and control units.    fatty acids or oils (101)    alcohol (102)    sulfuric acid (103)    alkylaromatics (104)    purified fatty acid methyl ester biofuel (106)    diluted acid (107)    reaction vessel (110)    sedimentation tank (111)    distillation apparatus. Another drawing (not shown) includes the washing stage for the alkyl ester-rich phase. (114)