The invention relates to a method and a device for preventing the blocking ofthebrew group, the filter holder or the coffee filter (8) in coffee machines whenusing fullysoftened water for the preparation of coffee. According to the invention, itis proposedto avoid the blocking of the machine by metering calcium and magnesium ions tothefully softened water. According to the invention this is achieved in that adevice with abag or reservoir (4) containing the calcium and magnesium ion containingsolution, ispositioned in a cartridge (3), which is arranged in the water tank of thecoffeemachine. The bag is in contact with the fully softenedwater via a capillary (5), which is closed with a check valve (6).
