Abstract: Laser Raman spectroscopy (LRS) was used to characterize the NiMoP impregnation solution and dried samples of NiMoP impregnated on γ-Al₂O₃. The effect of phosphorous content on the structure of the active phases in NiMoP impregnation solution and that during the impregnation process was studied. The LRS results indicate the active phases in NiMoP impregnation solution with lower phosphorous content are composed of several heteropoly compounds such as NixH6-2x\P2Mo5O23\, NixH7-2x\PMo11O39\ or NixH3-2x \PMo9O31\ and NixH3-2x\PMo12O40\. NixH6-2x\P2Mo5O23\ is dominant when phosphorous content increases. In addition, these active phases in NiMoP impregnation solution with lower phosphorous content are converted to seven poly-molybdate when the impregnation solution is impregnated into the cavities of γ-Al2O3, while higher phosphorous content hinders this conversion.