Method and apparatus utilizing successive reverse osmosis stages to produce highly purified water from potable water in a manner particularly suitable for use in a coin operated vending machine. The flow rates in the reverse osmosis stages are matched so that no storage tank is required between them. Operation of the purification stages is controlled by the level of purified water in a reservoir from which it is dispensed, and the level is automatically maintained within a predetermined range. A sump pan is mounted under the remainder of the machine to catch spillage, accidental overflow from the reservoir and the reject (brine) flow from the reverse osmosis stages, a sump pump is automatically actuated when the water reaches a predetermined level in the sump pan, and a valve at the feed water inlet is closed automatically in the event that the water level gets too high in either the reservoir or the sump pan. The machine will not accept coins in the event that the water in the reservoir drops to a predetermined level or fails to meet a desired standard of purity, and means is included for adding minerals to the purified water dispensed, if desired.