Abstract: The present invention relates to a method for the reactivation of phosphomolybdic acid based catalysts used for the conversion of saturated and unsaturated aldehydes to acids. The method comprises the step of feeding an oxide of nitrogen over the deactivated catalyst at a temperature of from about 100.degree. C. to about 400.degree. C. The method can be employed in the reactor by terminating the feed of reactant to the reactor, adjusting the reactor temperature as may be necessary to one suitable for reactivation and sweeping the reactor with a gas. Alternatively, the catalyst can be removed from the reactor for reactivation. Reactivated phosphomolybdic acid based catalysts are also disclosed. These catalysts have a ratio of molybdenum to phosphorus of 3:1 to 15:1 and are prepared by the process of feeding an oxide of nitrogen over the catalyst at a temperature of from about 100.degree. C. to about 400.degree. C.