The performance of ultraviolet (UV) reactors used for water treatment is greatly influenced by the reactor hydrodynamics. A detailed study of the hydrodynamic flow for two characteristic UV-reactor configurations with inlets concentric (L-shaped) and normal (U-shaped) to the main reactor axis was performed using particle image velocimetry (PIV). The inlet position was determined to have a dominant effect on the flow distribution in the reactors. The time-averaged flow structures in the L-shaped reactor exhibited a stable expanding jet behavior with zones of recirculation and low velocity flow. The flow was relatively insensitive to changes in operating conditions but was affected by alterations in the internal reactor structures. In the U-shaped reactor, the flow separation around the central annulus introduced an unsteady napping motion with higher velocities and short circuiting along one side of the reactor. For similar reactor dimensions and operating conditions, the L-shaped reactor is expected to provide a more uniform UV fluence and better reactor performance.