A method is developed to design a tank reactor for unique operation and for two simultaneous or consecutive reactions of the first order. Dimensionless groups are introduced which are either exclusively representative for the properties of the reaction system or exclusively for the design and operating variables. In a plot of the dimensionless slopes of the heat withdrawal line versus the dimensionless residence time in the reactor the region is indicated, where operation under conditions of uniqueness is feasible. The method is illustrated with two industrial examples; the oxidation of naphthalene and of ethylene. Whereas under conditions of uniqueness the naphthalene oxidation is shown to be feasible in a tank reactor, the ethylene oxidation cannot be executed economically in a tank reactor.