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Electronic self-quenching of Br2 (B) was investigated in the gas phase. The molecules were excited to individual rovibrational levels by a pulsed dye laser, and the fluorescence decay monitored in real time. Negativity curved Stern-Volmer plots were obtained for low J levels (<15) of v'=7, 11, and 14, and these were interpreted in terms of a deactivation mechanism involving collisional energy transfer to predissociated levels. The Stern-Volmer plots and resolved fluorescence data have been analyzed using detailed kinetic model of the quenching, energy transfer, and predissociation occurring within a collisionally coupled set of rovibrational levels. The model provided rate constants for quenching (4.2 + or - 0.5)X10 to the power cc molecule s rotational energy transfer 6 + or - 2)X10to the power cc molecules s, and indirect evidence for efficient near-resonant vibrational energy transfer (V-R, T). Electronic quenching of Br2(B) by He was also investigated. Previous studies gave rate constants of approx. x 0 to the -10 power CC molecule s, but in the present work it was found that the deactivation is entirely a consequence of energy transfer to predissociated levels. An upper limit of k sub 0 = 2x10 to the =12 power CC molecule s was determined. Keywords: Lasing Kinetics.