The possible preferential action exerted by an inhibitor on the transformation of one of two agonist substrates catalyzed by the same enzyme has recently been reported in studies on aldose reductase inhibition. This event was defined as "intra-site differential inhibition" and the molecules able to exert this action as "differential inhibitors". This work presents some basic kinetic models describing differential inhibition. Using a simple analytic approach, the results show that differential inhibition can occur through either competitive or mixed type inhibition in which the inhibitor prevalently targets the free enzyme. The results may help in selecting molecules whose differential inhibitory action could be advantageous in controlling the activity of enzymes acting on more than one substrate. (C) 2014 Elsevier Inc. All rights reserved.