Abstract To investigate the variation of effective atomic number ( Z eff) of alloys with photon energy, the Z eff of eight different alloys (tungsten steel, monel metal, solder, bell metal, bronze aluminium, bronze ordinary, platinum-rhodium-I and platinum-rhodium-II) have been studied for the total and partial photon interaction processes over a wide energy range from 10 keV to 100 GeV using a recent theoretical compilation by Berger and Hubbell (1987). For the total photon interaction, in all the alloys Z eff initially increases to the maximum value with increase in energy and then decreases to the minimum value with further increase in energy, after which Z eff again starts increasing with further increase in energy. The maximum and minimum value of Z eff is at different energies for different alloys depending upon the relative proportion and the range of atomic numbers of constituent elements of the alloy. For photo-electric absorption, Z eff increases in the low energy region and becomes independent of energy, whereas in Compton scattering, except below 200 keV, Z eff is constant up to 100 MeV. In the case of pair production, Z eff decreases with the increase in energy up to 10–12 MeV, after which it is noted to be independent of photon energy.