Insolubilsers are an essential part of coating formulations for paper and board which is to be printed by offset litho or likely to come into contact with water. Ammonium zirconium carbonate (AZC) is an inorganic insolubiliser which, in its stabilised form (Bacote 20), is becoming widely used because of the need for efficient formaldehyde free products. Bacote 20 is an alkaline solution of low viscosity which makes it easy to handle and meter in modern coating kitchens. Because of its nature it reacts rapidly on drying of the coating even under alkaline conditions. It is particularly effective with coatings based on soya protein and carboxylated latices where the chemical reaction with carboxyl groups is strong. Information is presented on the nature of ammonium zirconium carbonate, with the emphasis on Bacote 20. The mode of action is discussed and contrasted with that of other commonly used insolubilisers. The speed of reaction is a particular advantage of Bacote 20 and this is emphasised together with the effect of alkali on the performance. Advice is given on the practical use of the product in coating mixes to obtain maximum performance and avoid problems. Binder systems which work efficiently with Bacote 20 are illustrated especially those containing soya protein for board coatings. The effectiveness of the product in the presence of calcium carbonate is also highlighted. The rheology of coatings mixes containing AZC and Bacote 20 is discussed showing the advantages of Bacote 20 and the importance of correct pH control, type of binder and pigment.