Potassium impregnated mixed oxides of lanthanum and magnesium in 1:1 to 1:5 atomic ratios were prepared by co-precipitation at a constant pH. These were characterized by FEG-SEM-EDX, TEM, Powder XRD, Hammett indicators and acid/base titration and, were evaluated as heterogeneous catalysts for transesterification of used cotton seed oil with methanol. The optimized reaction conditions for the catalyst, prepared at pH 10 using 1:3 atomic ratio of lanthanum and magnesium, were the use of 1:54 molar ratio of oil to methanol and 5 wt.% of the catalyst at 65 degrees C that resulted into 96% conversion to biodiesel within 20 min. The catalyst showed tolerance of 1 wt.% of moisture. Concentration of potassium ions leached out from catalyst into biodiesel was within permissible limit of ASTM. (C) 2013 Elsevier Ltd. All rights reserved.