The incompressible unsteady flow past a channel with a pair of cylinders of rectangular cross section, placed in tandem normal to the flow, is investigated by numerical simulation. The objective is to evaluate the effect of cylinder separation distance, S / H, on the flow behavior and heat transfer on the channel walls, over a range of Reynolds numbers. Above a critical Reynolds number, these flows bifurcate to a time-periodic self-sustained oscillatory state. The results reveal for S / H = 2.0 three distinct flow patterns: steady flow ( Re = 200), time-periodic oscillatory state ( 400 < Re < 800), and quasi-periodic oscillatory flow ( Re = 1000). For S / H < 2.0 the intercylinder flow consists of a pair of steady counterrotating eddies that do not exchange fluid with the surrounding and unsteady eddies. Eddy shedding is observed only for the downstream cylinder for Re > 600.