Collisional removal of electronic energy from O-2 in the low-lying a(1)Delta(g) state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O-2(a(1)Delta(g), v = 1-3) in collisions with O-2 and CO2. Ozone photodissociation in the 200-310 nm Hartley band is the source of O-2(a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a(v = 1-3) levels in collisions with O-2 at 300 K is fast, with rate coefficients of (5.6 +/- 1.1) X 10(-11), (3.6 +/- 0.4) X 10(-11), and (1.9 +/- 0.4) x 10(-11) cm(3) s(-1) (2 sigma) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O-2(b(1)Sigma(+)(g)). With CO2 collider gas, the removal rate coefficient at 300 K is (1.8 +/- 0.4) X 10(-14) and (4.4 +/- 0.6) X 10(-14) cm(3) s(-1) (2 sigma) for v = 1 and 2, respectively. Despite the small mole fraction of O-2 in the atmospheres of Mars and Venus, O-2 is at least as important as CO2 in the final stages of collisional relaxation within the O-2 vibrational-level manifold.