Abstract: A tap-mounted water purification unit has a tap interface (14) for connecting it to a water tap, a reverse-osmosis filter (32), a pre-filter (30) arranged in the flow path before the reverse-osmosis filter (32), and a post-filter (34) arranged in the flow path after the reverse-osmosis filter (32). The reverse-osmosis filter (32) is located around a central duct (36) extending along a filter axis (8). The three filters (30, 32, 34) are arranged at different locations along the filter axis (8). The device can also be provided with a valve (64) for controlling the flow of water through it.