To investigate the variation of effective atomic number (Z,,) of alloys with photon energy, the Z,, 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,, 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,, again starts increasing with further increase in energy. The maximum and minimum value of Z,, 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,, increases in the low energy region and becomes independent of energy, whereas in Compton scattering, except below 200 keV, Z,, is constant up to 100 MeV. In the case of pair production, Z,, decreases with the increase in energy up to lo-12 MeV, after which it is noted to be independent of photon energy.