The codoping effect mechanism of Al and P on the solubility enhancement of Er3+ ion in SiO2 glass was clarified by electron spin-echo envelope modulation spectroscopy. It turned out that doped P ions preferentially coordinate to the Er3+ ion to form a "solvation shell structure", and the environment is similar to that in phosphate glass, while doped Al ions do not form such a selective solvation structure, taking octahedral coordination. This striking difference indicates that the primary roles of the P-doping and the M-doping are attributed to "enthalpy of mixing" and to "entropy of mixing", respectively.