Previously reported design criteria for cooled tubular reactors are based on the prevention of reactor temperature run away and were developed for single reactions only. In this paper it is argued that such criteri a should be based on the reactor selectivity, from which eventually a maximum allowable temperature can be derived. To this end and for the pseudo-homogeneous, one dimensional model of a cooled tubular reactor in which two parallel, irreversible first order exothermic reactions are carried out, two criteria are developed for the safe design and operation of the reactor. The criteria enable us to choose tube diameters and operating conditions, which are safe in view of the derived selectivity and of possible runaway as well. The method outlined can be used in the initial design stage and requires kinetic information on both the desired and the undesired reaction.