The kinetic and thermodynamic aspects of dyeing Thunderon SSN fibres (polyacrilonitrile fibres coated with a copper sulphide layer) have been studied and compared with results obtained on uncoated fibres. A considerable difference in the kinetic behaviour was found, and the apparent diffusion coefficients as well as the rate constants differ from each other by a factor of about three. At equilibrium the fibres react almost in the same way; their absorption isotherms were found to be quite similar.