A process for producing mixed esters of fatty acids as biofuel or additive to a petroleum fuel for use in a compression ignition (CI) engine. The process preferably provides a partial transesterification of a mixture of fatty acid methyl esters with at least one alkyl alcohol containing 2 to 8 carbon atoms in the presence of a heterogeneous solid acid catalyst to produce a mixture of the fatty acid methyl esters and alkyl alcohol esters of the fatty acids.