The determination of ammonium ions by the Berthelot reaction is reviewed in terms of reaction mechanism, and conditions, interferences, techniques (e.g., automated or radiochemical methods) and applications in the analysis of food, water, petroleum and gases and agricultural, clinical, metallurgical and pharmaceutical products. Although not generally applicable, the replacement of phenol by sodium salicylate is recommended and an alternative source of hypochlorite ion is proposed. Problems associated with high blanks caused by light-induced reactions involving nitroprusside breakdown products can be avoided by using aquopentacyanoferrate(III); however, its use as a catalyst makes the reaction more sensitive to pH. Although interference can occur from a variety of sources, this can be reduced by modification of the reaction conditions, and is only severe when there is a large ratio of interferent to nitrogen. (386 references).