A method for improving a biodiesel fuel comprising fatty acid methyl esters and free fatty acids, the method comprising adding to the biodiesel fuel an additive selected from the group consisting of a quaternary ammonium hydroxide, a quaternary ammonium alkoxide, and mixtures thereof, where the quaternary ammonium hydroxide has the formula selected from the group consisting of R 1 R 2 R 3 N + OH OH -, R 1 R 2 R 3 N + CH 2 CHR 5 OH OH - and R 1 R 2 R 3 R 4 N + OH - and the quaternary ammonium alkoxide has the formula R 1 R 2 R 3 R 4 N + O -, and mixtures thereof, where: R 1 and R 2 are independently selected from the group consisting of alkyl groups of from 1 to 18 carbon atoms, aryl groups of from 8 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, R 3 is selected from the group consisting of alkyl groups of from 2 to 18 carbon atoms, aryl groups of from 6 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, provided, however, that R 2 and R 3 may be joined to form a heterocyclic ring including the N and optionally an oxygen atom, R 4 is selected from the group consisting of hydrogen, alkyl groups of from 2 to 18 carbon atoms, alkylaryl groups of from 7 to 18 carbon atoms, -(CH 2 CH 2 O) n H, where n is from 1 to 18, where m and p are independently selected from integers from 0 to 18, except that the sum m+p is less than or equal to 18, and -CHR 5 CHR 6 Y, where R 5 and R 6 are independently selected from the group consisting of hydrogen, alkyl groups of from 1 to 18 carbon atoms, aryl groups of from 6 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, and Y is a non-acidic group selected from the group consisting of -OH, -SR 7 and -NR 7 R 8, where R 7 and R 8 are independently selected from the group consisting of hydrogen, alkyl groups of from 1 to 18 carbon atoms, aryl groups of from 6 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, and R 5 is selected from the group consisting of hydrogen, alkyl groups of from 1 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms. R 1 and R 2 are independently selected from the group consisting of alkyl groups of from 1 to 18 carbon atoms, aryl groups of from 8 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, R 3 is selected from the group consisting of alkyl groups of from 2 to 18 carbon atoms, aryl groups of from 6 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, provided, however, that R 2 and R 3 may be joined to form a heterocyclic ring including the N and optionally an oxygen atom, R 4 is selected from the group consisting of hydrogen, alkyl groups of from 2 to 18 carbon atoms, alkylaryl groups of from 7 to 18 carbon atoms, -(CH 2 CH 2 O) n H, where n is from 1 to 18, where m and p are independently selected from integers from 0 to 18, except that the sum m+p is less than or equal to 18, and -CHR 5 CHR 6 Y, where R 5 and R 6 are independently selected from the group consisting of hydrogen, alkyl groups of from 1 to 18 carbon atoms, aryl groups of from 6 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, and Y is a non-acidic group selected from the group consisting of -OH, -SR 7 and -NR 7 R 8, where R 7 and R 8 are independently selected from the group consisting of hydrogen, alkyl groups of from 1 to 18 carbon atoms, aryl groups of from 6 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms, and R 5 is selected from the group consisting of hydrogen, alkyl groups of from 1 to 18 carbon atoms and alkylaryl groups of from 7 to 18 carbon atoms.