Carbon-14-labelled tetradecyl ketene dimer and hexadecyl hexadecanoate have been synthesized and used to study the mechanism of neutral sizing. The development of sizing by these compounds is very sensitive to the degree of post-sheet-formation heat treatment. Both compounds can be extracted from the sheets in an unreacted form by an inert solvent but, in the case of the dimer, a small (less than 5%) amount of residual dimer remains and is able to promote extensive sizing. Beta-radiographs of sized sheets show that considerable diffusion and migration of the dimer takes place as a result of heat treatment. -- AATA