Total attenuation cross sections in six alloys at 662-keV photon energies are measured by a transmission method using a NaI(Tl) scintillation spectrometer. The cross sections for the photoelectric process and for coherent scattering are deduced using theoretical cross sections taken from recent publications and are subtracted from the measured total attenuation cross sections to get the incoherent scattering cross sections. Finally, effective atomic numbers for the total gamma-ray interaction and for the incoherent scattering process are obtained by interpolation from graphs of the respective cross sections versus atomic numbers. The results are compared with effective atomic numbers estimated using semiempirical expressions.