The invention relates to a process for the synthesis of monochloramine by reaction of an ammonium chloride solution with a sodium hypochlorite solution. The sodium hypochlorite solution is basified beforehand with an inorganic base and the ratio of the concentration of ammonium chloride in the reaction medium to the concentration of sodium hypochlorite in the reaction medium is between 1 and 1.5. This ratio is preferably equal to 1.1.