A number of reaction processes of the fine chemical and pharma industry undergo selectivity losses, since the target product (often a low volatility compound) can be further consumed by a nonselective reactant, according to a mixed series/parallel kinetic scheme. In these cases, performing the reaction in a semibatch recycle reactor (SBRR) allows achieving both a high process selectivity and productivity even in a noncontinuous plant, since the target product concentration in the reactor can be limited adopting a relatively low excess of the selective reactant. In the literature, on the basis of a mass balance approach, a number of boundary diagrams have been developed that allow to predict selective and productive operating conditions for SBRRs in which (1,1) order reactions occur. However, when dealing with fine chemical and pharma reaction processes, the estimation of the kinetic parameters is often not practicable, due to the huge variety of productions encountered. In this work a criterion has been developed that allows for identifying the optimal operating conditions of SBRRs without knowing the kinetics of the reactions involved. The criterion has been applied to a reaction process of industrial interest performed in a SBRR, that is the production of ethylene glycol from ethylene oxide, with further ethoxylation of the desired product to diethylene glycol. (C) 2015 Elsevier Ltd. All rights reserved.