Al~traet--An attempt has been made to theoretically model the chemical response of the Fricke dosimeter using track structure concepts and to indicate areas in which the modelling can be improved. The Monte Carlo technique has been used to investigate the differences in the fundamental chemical response of the ferrous sulphate Fricke dosimeter to photon and electron irradiation. Calculations of the fractional energy loss in the track entities (spurs, blobs and short tracks) were made, together with more quantitative descriptions of these clusters which were used to estimate the final ferric yield. Photons and electrons of various energies were considered for an infinite medium and for a water equivalent ampoule in a water scattering medium, using four secondary electron energy distributions. Emphasis was placed on the variation in yields rather than in the absolute values. The results show that the relative differences in the Fricke G-value, not only between electrons and photons, but also with primary energy are critically dependent on the variation in the mean energy of the secondary electron following ionisation with primary energy. This dependence is due to an expected strong variation in Fricke yield for low energy electrons (< 100 eV).