UV photodissociation of OCS via 2A'((1)Delta) leads to a bimodal speed distribution of S(D-1(2)) fragment atoms whereas a single speed distribution characterizes the S atoms emerging from the 1A"((1)Sigma(.)) surface. The potential energy surfaces of these two states are almost degenerate, meaning that the bimodal state distribution observed for the 2A' state cannot be explained by the topography of the adiabatic surfaces. It is shown that the bimodal distribution originates from concomitant adiabatic and non-adiabatic dissociation processes from the 2A' state. The non-adiabatic transition is due to unlocking of an S atom electron orbital from the C-S axis by rapid rotation of CO.