Experimentally determined mass transfer coefficients for wood surfaces are often much lower than theoretically and experimentally determined coefficients for other surfaces. An investigation of this is made by comparing mass transfer coefficients for water and saturated non-wood surfaces, with coefficients measured for wood surfaces. It is found that measurements on wood surfaces are usually evaluated by assuming that Fick's law describes diffusion in wood. As this is not always true, it is proposed that the mass transfer coefficients measured on wood surfaces are low because the internal mass transfer in wood is not well understood. New measurements are presented which support this conclusiou.