The influence of molecular size and shape and the impact of ionic character on the diffusivity of dyes into human hair fibre have been investigated using advanced molecular modelling techniques. A model has been proposed consistent with current theories on the pathways followed by molecules as they penetrate into the hair fibre and inspired by apparent similarities between the mechanisms involved in hair dyeing and in enzyme and zeolite chemistry. A previously reported method has been extended to take more realistic account of the mechanism of the diffusion process in hair fibres and the motion of the hair dye molecules as they enter the fibres. Original data have been reanalysed mathematically and the interpretation of the results refined. A revised descriptor, LD, the longest dimension of the smallest cross-section of the optimum parallelepiped enclosing the molecule, is proposed as a measure of the diffusivity of the dye into the fibre. Molecular size limit values for dye penetration are proposed varying with the charge on the hair dye molecules. © 2008 The Author. Journal compilation © 2008 Society of Dyers and Colourists.