The experiments described here differ from previous experiments on ionisation in alkali-metal vapours in that it is possible to measure the m/e ratio for the ions produced. Three methods of producing ions have been employed: photo-ionisation, ionisation by electron impact, and “ionisation” by the attachment of K + ions to neutral potassium atoms. In the following we shall first describe the experimental methods, next give the results obtained by each method of ionisation with a brief discussion on each, and then proceed to a general discussion of the whole of the results.