In a recent paper experimental data on total deposition of monodisperse aerosols have been summarized for a wide range of particle sizes and breathing conditions (Heyder et al., J. Aerosol Sci.17, 811–825, 1986). Particle deposition was determined during steady breathing by either evaluating the number of inspired and expired particles per breath with an online technique or by measuring the mean particle concentration in inspired and expired aerosols under equilibrium conditions a certain distance away from the entrance of the respiratory tract. In the present paper the effect of the instrumental dead space on experimental data for total deposition is investigated for both kinds of inhalation apparatus. It turns out that in an online-system the effect of instrumental dead space can be neglected as long as its volume does not exceed 10% of the tidal volume. In an equilibrium concentration system, on the other hand, the instrumental dead space results in a systematic error of total deposition data, which depends on tidal volume and amounts up to 10% for a tidal volume of 500 cm3.