Abstract: A compact reverse-osmosis water purifier especially adapted for use in the household on a counter-top, with an easy-to-operate diverter valve to connect the purifier to a water faucet and sink. A battery-operated pump is provided to dispense water from a storage reservoir. This reduces the size of the purifier unit while maximizing the height from the dispensing spout to the counter-top to accommodate relatively tall vessels for receiving the dispensed water. An automatic conductivity test device causes a red or a green lamp to be lit each time a button is pressed to dispense water, the green lamp indicating that the water is of sufficient purity, and the red lamp indicating that it is not, or that the device is malfunctioning or requires flush cleaning. The diverter valve can be operated to selectively pass water through the faucet in the usual way, or to feed water to the purifier.