Migration behavior of epoxidized neem oil (ENO) from poly(vinyl chloride) (PVC) plasticized with ENO to acrylo nitrile butadiene rubber (NBR) was investigated. Central composite rotatable design (CCRD) for three variables at five levels was chosen as the experimental design. Concentration of Neem oil in PVC matrix, acrylo nitrile (ACN%) content in NBR matrix, and peroxide content used in NBR formulation were selected as three independent variables. Other parameters pertinent to migration phenomena such as partition coefficient, activation energies, rate constants, and diffusion coefficients were evaluated and these properties were fitted to a distinctive response equation generated by CCRD. Proposed response equations for most parameters were considered as having sufficient explanatory power and "good fit" in the statistical sense. Partitioning of ENO as a plasticizer between PVC and NBR follows first order equilibrium kinetics, and the forward reaction is found to be endothermic. The percentage migration of plasticizer at 296 K at equilibrium is well below 25% for the majority of cases irrespective of other pertinent factors. Parameters such as forward, backward, and overall rate constants and their respective activation energies and enthalpy change related to the partitioning phenomenon are influenced to different degrees by plasticizer concentration in PVC, ACN% in NBR and crosslink density of NBR. The influence caused by plasticizer concentration and ACN% was found to be the most significant. Diffusion of plasticizer through PVC matrix is the rate determining step for the overall migration process. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 823-838, 2011