Ce3+-doped silica nanoparticles dispersed within pores of mesoporous silica were synthesized by soaking and sol-gel technique. Liquid-like absorption and fluorescence have been observed. What is important is that the emission intensity increases with the exposure time to humid air and reaches saturation (which is about four times of the original value) at about 5 h. Subsequent drying leads to a decrease of the emission intensity to the original value. Reversible emission enhancement is obtained by alternately exposing and drying the sample. This is attributed to the formation of Ce3+ ion aqueous solution within pores of the sample during its exposure to air, due to the nonparticipation of Ce3+ ions in the silica network.