Method of concentrating aqueous alkali and apparatus suitable for this purpose. A very energy-saving method of concentrating aqueous alkali originating, for example, from a chloralkali electrolysis plant and an apparatus suitable for this purpose are described. The method/the apparatus utilizes heat of reaction from the formation of 1,2-dichloroethane and includes multistage concentration of the aqueous alkali, where at least part of the heat required for concentrating the aqueous alkali originates from the plant for preparing 1,2-dichloroethane and at least a further part of the heat required for concentrating the aqueous alkali originates from at least one of the higher stages of the plant for concentrating the aqueous alkali and is used for partial heating of the first stage. The apparatus can be used for retrofitting existing integrated plants made up of a DCE plant and chloralkali electrolysis or in the erection of new plants.