Abstract: The stability constaut of Tl⁺complex of 18-crown-6 iu methauol solution was determined by the fluorescent quenching of Tl⁺.Those of Na⁺,K⁺and La³⁺complexes were determined by competitive coordination.The values of logKfor Na⁺and K⁺measured by this method were found in excellent agreement with those determined potentiometrically using ion selective electrodes.The value of logKfor La³⁺was found to be higher than that determined calorimetricallg by Izatt.It is proposed that this discrepancy may be caused by the difference of water content in me-thanol.