This paper analyzes the miscibility state of Poly(L-Lactide)(PLLA) and Poly(DL-Lactide) (PDLLA) with Poly (styrene) (PS) and Poly(vinyl phenol) (PVPh) by means of Molecular Dynamics (MD) simulations performed using the COMPASS force-field. Immiscibility was found in polylactide/PS blends while miscibility was the result in polylactide/PVPh blends, both previsions agreeing with the experimental behaviour. The values calculated for the Flory-Huggins interaction parameter, x, have been compared with the experimental results and with estimations based on existing miscibility models Even though the dependence of x with composition and the prediction of miscibility is correct, both the solubility parameters and the interaction parameters obtained prove that molecular modelling tends to underestimate the strength of the interactions. The observed differences are explained in terms of the absence of cooperativity effects for oligomeric chains used in molecular modelling of high molecular weight polymer blends (C) 2010 Elsevier Ltd All rights reserved.