This invention relates to a reactor 1 for the continuous production of linear or cyclic acetals, in particular trioxane, dioxolane, tetroxane, dimethoxymethane, diethoxymethane, diethoxyethane and dibutoxyethane. This reactor has internal adjacently arranged evaporator elements 2, which allow for flow-through of a heat transport medium, and which are at least two meters high so as to generate a thermosiphon flow. The clear cross section between the evaporator elements is 20 to 80% of the overall cross section. Using the reactor, circulation ratios of up to 250 can be achieved. Thus, the reliability and availability of a production plant is increased while the susceptibility to breakdown decreases.