We found that Ce(III) ion is oxidized to form Ce(IV) ion on a quartz cell in air-saturated organic solvents, causing a strong absorption that overlaps with the absorption of the Ce(III) complex. Due to the strong absorption, the emission quantum yield (Φ em ) of the luminescent Ce(III) complex was under-estimated. A silane treatment on the quartz cell, which prevents the formation of the Ce(IV) species even in air, and titration experiments under an Ar atmosphere enabled correct evaluation of the luminescent properties where the Φ em of an electrically neutral Ce(SCN) 3 reached 91 % in acetonitrile.