We demonstrate that Q I membranes significantly hinder diffusion of solutes based on sizeexclusion, though in ambiguously defined pores. These tortuous hydrophilic pores interrupt solute dynamics, and we propose that the chemical heterogeneity within the pore regions contributes to the high solute rejection seen experimentally. The instantaneous, restrictive pore diameters can be as narrow as 0.2 nm, which in theory would reject almost all solutes. However, the head groups creating these pore diameters are highly mobile. We conclude that the soluteaccessible pore diameters are thus also partially described by total diameter occupied by hydrophilic groups within the membrane. The flucutuating nature of the pore walls results in larger solute-accessible regions than those that are purely solvent alone, which contributes to the permeability of larger solutes seen experimentally.