The primary focus of this work was to determine the relationship between chemical composition and the Zener-Hollomon exponent q in the peak strain equation for hot working of C-Mn steels with the ultimate aim of improving the modelling of mean flow stresses (MFS) for hot strip mills. Therefore, the hot deformation behaviour of C-Mn steels was examined in which the C and Mn contents were increased systematically between the levels 0.035 up to 0.52%C and 0.22 to 1.58%Mn respectively. In addition, data from other workers were also analysed to complement the results from this investigation. As opposed to the observations from some other workers that the apparent activation energy for hot working Q HW appears to decrease with an increase in the carbon content, it was found that, despite a weak relationship, Q HW increases with an increase in carbon content i.e. from 300 to 355 kJ/mol as was also found by others. Consequently, the Zener-Hollomon exponent q in the peak strain equation for hot working and hence also the critical strain for dynamic recrystallisation (