Formulations in water suspension of microcapsules based on 3-iodo-2-propynyl butyl carbamate (IPBC) comprising (parts by weight): (a) 10-60 parts of polymeric microcapsules comprising inside them IPBC and a synergizing agent formed of one or more alkylbenzenes having a number of carbon atoms from 9 to 20; (2) 1-5 parts of one or more dispersants; (3) 1-20 parts of one or more excipients selected from thickeners, antifoam, antifreeze, in can preservative agents; (4) water to 100.