A theoretical treatment is given of ’’partial’’ sinks, i.e., those in which the amount of a diffusing species removed through a surface is proportional to its concentration there. The effect of the existence of a partial sink on the concentration profiles and on the amount of material which accumulates on the surface is treated. Diffusion coefficients calculated from measured data will be in error if a perfect sink is assumed when a partial sink actually exists. The magnitude of this error is considered.