The simple T-piece is frequently used as a weaning system during respirator treatment. It is modified with an expiratory non-compliant reservoir (EnCR), an inspiratory compliant reservoir (ICR) and/or an expiratory one-way valve. The distribution of expiratory gases and rebreathing were studied in a model set-up in the corresponding systems at different fresh gas flows (FGF) and tidal volumes (VT). An EnCR produces no change, whereas an ICR causes the expiratory gas to flow into the inspiratory limb, an effect which is intensified by the presence of an expiratory valve. With a falling FGF and a rising VT, increasing amounts of expiratory gas are found in the inspiratory limb in the modifications with an ICR. However, this gives rise to rebreathing in the valve modification with a low FGF and high VT. The modification of the T-piece with an ICR but without a one-way valve is advantageous, as this system combines only slight dilution via the expiratory limb and a minimal risk of rebreathing.