A process for a continuous production of chlorine dioxide in a cooled reaction vessel subjected to an overpressure, comprising continuous addition of an aqueous solution of an alkali metal chlorate, sulfuric acid and sulfur dioxide to the reaction vessel in proportions to generate chlorine dioxide at a temperature from about 30.degree. to about 60.degree. C. and at an acidity of about 5 to about 11 N. The reaction solution containing dissolved chlorine dioxide is continuously withdrawn. At the start up of the process the reaction vessel is filled with the water solution of alkali metal chlorate, optionally containing alkali metal chloride, and the sulphuric acid with a feed concentration of from about 20 to about 60 percent by weight whereafter the flows are adjusted to correspond to the production rate. Finally sulfuric dioxide is added so slowly that no gas phase is formed and the sulphuric acid flow is slowly increased until the suitable flow for the chosen production is reached.