Abstract A systematic study of the quenching of Ba( 1P 1) fluorescence by CH 4 in Ar clusters has been performed for different argon cluster sizes and numbers of methane molecules in the clusters. The quenching rate as a function of the number of methane molecules in a given cluster follows a first-order rate law. Quenching rates for different cluster sizes have been deduced from the experimental data. The average time for Ba( 1P 1) to collide with CH 4 and be quenched is 18±5 ns in an Ar 1000 cluster. The quenching time varies with the cluster size as n 2 3 .