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An experimental method, employing a fast population perturbation technique, is described to measure the vibrational-vibrational ( ) collisional probability of a diatomic molecule for large vibrational quantum numbers and υ. The relaxation of the perturbed gain of a pair of vibrational levels is a function of the vibrational populations and rate constants . The numerical inversion of the master rate equations determining this relaxation does not give unique value for (or ), but lower bounds can be evaluated and with empirical formulas, having several adjustable constants, it can be shown that probabilities of the order of unity are required to satisfy the experimental data. The method has been specifically applied to the CO molecule, but other molecules such as HX(X = F, C1, Br), NO, etc., could also be measured.