Abstract The collisional deactivation of vibrationally highly excited ethylene molecules produced in the 121.6 nm photolysis of ethyl bromide has been studied at 298 K in the presence of He, CF 4, CO 2 and C 2H 5Br as collisional partners. By observing the ratio of stabilization to decomposition ( S/ D) of vibrationally excited ethylene and by comparing the observed values with the theoretical curves, the average energies 〈Δ E〉 down removed per collision were deduced for various collision partners. The 〈Δ E〉 down for a stepladder model were found to be 6500 cm −1 for C 2H 5Br, 1700 cm −1 for CF 4 and 1700 cm −1 for CO 2. In the case of an exponential model, the 〈Δ E〉 down was 700 cm −1 for He. The relative collision efficiency β was measured as a function of the total pressure of the system, and the values extrapolated to high pressure (ω → ∞) were 0.41 for He, 0.83 for CO 2 and 0.83 for CF 4. A scheme of energy partitioning among photofragments has also been proposed. Our results indicate that the molecular complexities of the collision partners play a role in the energy transfer by the bath gases. The present work describes a successful approach to measuring 〈Δ E〉 down values for various bath gases, by studying the secondary process of the C 2H 5Br photodecomposition reaction at 121.6 nm.