We report about the physical and chemical origin of the quick loss of intensity of certain argon/mercury cold cathode illumination tubes, especially those manually coated with luminous powder using the European powder technology. The reason is the disintegration of the phosphoric acid binder, which leads to the release of water vapor in the operating plasma tube. Subsequently formed mercury-water complexes lower the intensity of the UV emission of the Hg discharge in the 250 nm range, because the complexes decay radiationless into the ground state, thus, reducing the brightness of the fluorescent powder in the visible range substantially. If shut off, the respective tubes show a turn in color of the luminous powder coating from white to a light brown. We also found that the released water vapor causes heavy sputtering of the activated hollow-cathode-type nickel electrodes. Possible reasons for this are discussed, too.