An ultrasensitive millisecond detecting system has been developed to study the O 2 ( 1 Δ g ) dimol luminescence in solution. In all solvents studied the O 2 ( 1 Δ g ) dimol emission intensity is quadratically proportional to the excitation energy and its decay dynamics correspond to twice the decay rate of 1 O 2 ( 1 Δ g ). The intensity ratio for the 0,0 vs. 0, 1 bands was found to be O 2 ( 3 Σ - g ) concentration dependent, suggesting that in addition to the emission involving two colliding O 2 ( 1 Δ g ) molecules, the interaction between O 2 ( 3 Σ - g ) and 1 O 2 dimol may also play a minor role to account for the simultaneous transition.