A population balance analysis of the statistical distribution of substrates over reversed micelles indicates the importance of accounting for concentration fluctuations within the micelles in interpreting observed enhancements in enzymatically-catalysed reactions in reversed micellar media. When the reactions are slow relative to the rates of solubilizate exchange via micellar coalescence and redispersion processes, analytical solutions to the detailed population balance equations are obtained, and the connection between Michaelis-Menten kinetics in reversed micellar media and bulk aqueous solution is made. It is shown that the finite occupancy number of the micelles at low water content leads to a statistical distribution of substrates over the micelle population, and that this can give rise to changes in the apparent reaction kinetics, even in the absence of changes in the intrinsic reaction parameters themselves. The model predictions are in good qualitative agreement with reported experimental results.