The qucnching of the eIcctronicaIly excited strrtes of organic mo!ecu!es by free radiwls such ;IS nitric oxide and oxygen was kt investigttd in any detail by Bawzn and Net&f [ 11. and Itas since then received incrrxsing attention as attempts have been made to un-Axstand this probicm from both the thcoreticai and the cxpcrinrcntal side_ The question of the mechanism by which this yuetxhin~ occurs is ;i rather compkx one, since it is possible to envisage a nunlber of p&ways Irrding fo the observed quenching. proceeding. for esztmple, by an cschzu~~e mechanism or by an energy transfer mechanism involving spin conservation. or by some combination of these_ The work to be reported here dtxds with the quenching in a polrtr solvent of excitixi sitigkt st3tes of wom;lIic mokcuksby moiecuhr oxygen. Quenching by oxygen is generally diffusion amtrulkd. showing in most cases IittIt vadation in rztc on ch;lnging tite moIecuIc being quenched [3-i _ Iicweuer. it appears to have been little studied in polar solvents_ Pohr sdvats ;1:e in this respect particuktrly inkresting, because. in addition to the mechanisms mentioned abow, there arc the additional possibilities of ctec:ron transfer from the exited motecule to O,, znd quenching by coupling to chzrse transfer states_ An invcstigztion of Cluorecen~~ quenching by osygen in acutonitrik is therefore of some interest, since acetonitrik is 3 soivent which is IiigIiIy polar and in which x0 PHYSICS LC-ITCRS