A number of exothermic semibatch (SB) processes of the fine chemical industry may undergo selectivity drops because of the triggering of consecutive degradation reactions of an intermediate target product. The rate of such reactions is normally negligible at the operating conditions,specified by the chemical recipe; in fact, the selectivity loss occurs typically under upset conditions in which both reactions (that is, the main one and the consecutive one) are triggered. This undesired event may lead to dangerous temperature increases if the consecutive degradation of the target product is relatively exothermic. In this work, on the basis of a fully available set of process variables for calculating a suitable Key Process Indicator, a simple kinetic-free criterion is presented, through which the marginal ignition of a potentially dangerous consecutive reaction can be early detected: this allows for a prompt corrective action aimed to suppress the undesired event, hence recovering the process safety and selectivity. The criterion has been validated using literature data about a relevant industrial oxidation reaction of the fine chemical industry: the selective oxidation of 2-octanol to 2-octanone.