Direct numerical simulations (DNS) of passive scalar mixing in isotropic turbulence is used to study, analyze, and, subsequently, model the role of small (subgrid) scales in the mixing process. In particular, we attempt to model the dissipation of the large‐scale (supergrid) scalar fluctuations caused by the subgrid scales by decomposing it into two parts: (i) the effect due to the interaction among the subgrid scales,E≫&fgr;; and, (ii) the effect due to interaction between the supergrid and the subgrid scales,E≳<&fgr;. Model comparison with DNS data shows good agreement. ©1996 American Institute of Physics.