Calculations are presented of the phenomenological coefficients defined in the non-equilibrium thermodynamics description of the fluxes of solute atoms and interstitial defects arising from concentration gradients in a dilute f.c.c. alloy [A. R. Allnatt el at., Acta metall. 31, 1307 (1983)]. The method illustrates the extension of earlier analyses of linear response formulae for the coefficients to models with dumb-bell interstitials. Improved results are given for the coefficients in terms of jump frequencies for the kinetic model studied earlier. In the limit of strongly bound interstitial-solute pairs the cross-section characterizing coupling of the two fluxes differs from the earlier result, but is now in qualitative agreement with a previous intuitive estimate for certain combinations of jump frequencies. For the numerical values of jump frequencies typical of copper the values of phenomenological coefficients as functions of relative solute size differ negligibly in qualitative behaviour from those given earlier.