In this paper, the effective atomic numbers (Zeff) and electron densities (Neff) representing interaction of gamma rays with oxides of lanthanides were studied. The effective atomic numbers for photon energyabsorption (ZPEAeff), photon interaction (ZPIeff), relative to air ZReff, the effective electron densities for photon energy-absorption (NPEAeff) and photon interaction (NPIeff) were calculated using the values of the mass attenuation and energy absorption coefficients. In the continuous energy region, agreements and disagreements were observed between photon interaction and photon energy-absorption for Zeffs and Neffs of compounds in different energy regions. In addition, absorption edge effects on Zeffs leading more than a single value of Zeff at a specific energy have been discussed for the given compounds. Comparisons with experiments wherever possible have been carried out for calculated values of Zeff and Neff.