The relationship between Lagrangian micromixing models, which are widely employed in chemical reaction engineering, and Eulerian computational fluid dynamic (CFD) models based on the Reynolds-averaged species conservation equation is explored. A general modeling methodology which combines the strengths of both approaches is developed in the form of a multi-environment CFD micromixing model. The formulation is shown to be equivalent to a presumed multi-scalar probability density function (PDF) approach. The four-environment generalized mixing model (GMM) model originally proposed by Villermaux and Falk (Villermaux and Falk, Chem. Eng. Sci. 49 (5127) (1994)) is used to illustrate the methodology by applying it to model a series-parallel reaction in a tubular reactor. (C) 1998 Elsevier Science S.A. All rights reserved.