ReceiVed: August 19, 1997 In his Comment1 to our paper,2 Vanag revives the once current view4 that the system inhomogeneities that give rise to stirring effects in nonlinear reactions are due to internal, statistical fluctuations in particle density, rather than to other extrinsic sources of inhomogeneous noise, and he bases his claims on results of a calculation using a probabilistic cellular automaton model.3 All cellular mixing2 models are mesoscopic and unsuited to deal realistically with the concentration fluctuations that occur on a molecular scale and that are the sources of nucleative dynamics. By treating a mesoscopic cell rather than molecules as the elementary, fluctuating entity, one jumps over many orders of magnitude in particle number N and in the relative amplitude A ) N-1/2 of fluctuations. Let us look at a specific example: consider first a reactive fluid with concentrations of the order of 1 mM ≈ 6.7 × 1017 molecules/cm3. And consider also a 2-D mesoscopic cell model with N2 ) 100 × 100 ) 104 cells of a size that is of the order of the Kolmogorov length LK ≈ 10 μm. It describes a subvolume with side length 100 × 10 μm ) 1 mm, where a reference area (volume) of 1 cm2 contains 106 cells/cm2. In 1 cm3 of the molecular fluid the relative fluctuation amplitude is Amolec ) 1.2 × 10-9; in 1 cm2 of the 2-D cellular fluid it is Acell ) 10-3. Hence, the cellular model predicts fluctuations that are 1.2 × 106 times bigger than those on the molecular level. In view of this it is understandable why Vanag’s CA model weighs statistical fluctuations so heavily. In a CSTR it even predicts that they dominate over the inflow-induced inhomogeneities. The “real”, molecular fluctuation amplitude Amolec is probably too small4 to outweigh other likely sources of system inhomogeneity. The best understood case is that of the CSTR,5 for which we have now quantitative evidence6 supported by simulations and experiments6,7 that confirms the earlier view8 that the primary inhomogeneity, responsible for stirring effects in bistable systems, arises from the forcing of the CSTR by the inflow stream. For systems with one dynamical variable, the intensity σ2 of the reactor inhomogeneity, which is related to the observed fluctuation intensity, scales as6,7b