Abstract: A heterogeneous catalyst for use in esterification and/or transesterification reactions is provided having the formula AxB2-xO4-x, where x is between 0.25 and 1.2, A is selected from calcium, magnesium, strontium, or barium, and B is selected from manganese, cerium, titanium, zirconium, silicon, tin or germanium. The heterogeneous catalyst is formed by co-precipitating the corresponding nitrates of the catalyst materials with ammonium carbonate to form a precipitated product which is then calcined. The heterogeneous catalysts can be used to make mono-alkyl ester fuel from feedstocks containing waste oils, fats and grease in the presence of water and/or free fatty acids.