A process for production of chlorine dioxide which is substantially free of by product chlorine. Chlorine-dioxide is produced from an alkali metal chlorate, a mineral acid and a reducing agent such as methanol in a reaction medium maintained at an acid normality of less than 9 and with a high chlorate molarity. The process is carried out in the substantial absence of additional chloride ion being fed to the process. It has been found that increasing the chlorate concentration at a given acid normality reduces the amount of chloride in the reaction medium thus reducing the amount of chlorine by-product. The process is carried out in a vessel operated under subatmospheric pressure, whereby water is evaporated and withdrawn together with chlorine dioxide and the alkali metal salt of the mineral acid is crystallized within the reaction vessel and withdrawn therefrom. According to the invention, high production rates with high efficiency of chlorine dioxide can be achieved while producing essentially no chlorine by-product.