NOVELTY - A solid acid catalyst is new. It has a total content of sulfur and phosphorus of 3-10 wt.%; a surface area of 50-200 m2/g; a pore volume of greater than or equal to 0.15 ml/g; and an average pore diameter of greater than or equal to 40 Angstrom .    USE - The solid acid catalyst of (I) is used for the esterification of carboxylic acids, particularly mixtures of carboxylic acids and in the manufacture of esters from alcohols and carboxylic acids (claimed). It is used in the manufacture of biodiesel.    ADVANTAGE - The solid acid catalyst of (I) has high activity for biodiesel synthesis at 100-200, preferably 120-180 degrees C and 100% selectivity towards the formation of alkyl ester, where despite the high acidity, no ethers or other by-products are detected, even when the alcohol excess is up to 5 times the stoichiometric amount, at 120-180 degrees C; is a multi-substrate, exhibiting similar activity for fatty acids in the series 12-18C and for normal and branched alcohols in the series 1-8C; keeps almost constant activity over longer period when the water concentration in liquid phase is kept low, preferably below 2 wt.%; and keeps its integrity over long operation periods, provided that the reaction temperature is below 180 degrees C and the water content in the organic liquid phase is below 2 wt.%. It has large pores, highly acidic active sites, and tunable surface hydrophobicity.    DETAILED DESCRIPTION - A solid acid catalyst of formula M1aM2bOx(AO3H)y (I) is new. It has a total content of sulfur and phosphorus of 3-10 wt.%; a surface area of 50-200 m2/g; a pore volume of greater than or equal to 0.15 ml/g; and an average pore diameter of greater than or equal to 40 Angstrom .    M1=transition metals, indium, and/or tin;    M2=transition metals, lanthanide metals, indium, and/or tin;    A=S and/or P;    a, b, y=0.01-1;and    x=1-4.    INDEPENDENT CLAIMS are included for:    (1) a method of manufacturing a particulate solid catalyst of (I); and    (2) a process for the esterification of carboxylic acids, particularly mixtures of carboxylic acids, comprising reacting carboxylic acid with an alcohol in the presence of solid acid catalyst of (I).