reaction by adding restricted in terms of time and amount starting material mixture to the circulating reaction mixture in such an amount that the starting materials can no longer be detected in the part of the circulating reaction mixture taken off downstream of the reactor the addition of starting material mixture is preferably repeated until all the reaction mixture being circulated has reached the reaction temperature the method of the invention can be used for example for the start up of the cleavage reaction in the cleavage of cumene hydroperoxide to form phenol and acetone for the purposes of the present invention reaction mixtures can be either flowable solid liquid or gaseous materials or combinations of liquid flowable solid and or gaseous materials the reaction mixture can include for example starting materials products catalysts solvents and or inert gases loop reactor systems can for the purposes of the present invention include systems which have at least one reactor whose contents are preferably at least partly circulated in loop reactor systems part of the reaction mixture which may include predominantly starting materials or predominantly products preferably predominantly products of at least one reaction that is to take place in the loop reactor system is preferably circulated via at least one reactor predominantly is preferably understood to mean at least 50 more preferably at least 60 and most preferably at least 70 of starting materials or products based on the weight or volume of the reaction mixture to circulate the reaction mixture it is preferable to use equipment such as pumps or blowers at.