Partitioning of ethylene glycol and its polymeric forms into the pore of the volume-sensitive outwardly rectifying (VSOR) anion channel was studied to assess the pore size. Polyethylene glycol (PEG) PEG 200-300 (Rh = 0.27-0.53 nm) effectively suppressed the single-channel currents, whereas PEG 400-4000 (Rh = 0.62-1.91 nm) had little or no effect. Since all the molecules tested effectively decreased electric conductivity of the bulk solution, the observed differential effects between PEG 200-300 and PEG 400-4000 on the VSOR single-channel current are due to their limited partitioning into the channel lumen. The cut-off radius of the VSOR channel pore was assessed to be 0.63 nm.