A method is described for the preparation of a supported metal nitrate, suitable as a precursor for a catalyst or sorbent, comprising the steps of: (i) impregnating a support material with a metal nitrate, (ii) optionally drying the impregnated material at low temperature, and (iii) exposing the impregnated material to a gas mixture comprising nitric oxide at a temperature in the range 0-150 DEG C, to form a dispersed supported metal nitrate. The metal nitrate may subsequently be converted to the corresponding oxide by calcining the metal nitrate to effect its decomposition. Preferred metals are iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper or a mixture thereof.