The relaxation of vibrationally excited CS, formed in the flash-initiated reaction: O(3P)+CS2→ SO + CS, has been studied by monitoring the absorption of the A1Π—X1Σ+(2,1) band photoelectrically. Rates of decay were measured in the presence of various added gases, and hence rate constants determined for the de-excitation of CS(ν= 1) by ortho-H2, para-H2, HD, 3He, D2, 4He, N2O, CO2, H2O, D2O, H2S and D2S. The much greater efficiency of N2O compared to CO2 shows clearly how the probability of vibration-vibration energy exchange is enhanced if both species are infra-red active. Vibration-rotation energy transfer may occur with the collision partners which have small moments of inertia. Where appropriate, experimental transition probabilities are compared to those predicted by Sharma's recently published theory.