The equation for pick-up of surface sizes on the size press recently proposed by Hoyland & Howarth contains three terms. These are the immobilisation term and the absorption term, which are controlled by the sheet and the hydrodynamic term, which is primarily a function of speed. This paper describes pilot plant experiments to establish how the immobilisation and absorption terms are related to the various aspects of the sheet structure and composition . The most important factors were found to be structural differences associated with freeness changes and the presence of very small quantities of resin size. The latter were important at levels well below those associated with normal size addition.