In growing crystals of synthetic fluor-phlogopite, a trace amount of platinum might be found in them because of the use of platinum vessel.A study is made on spectrographic method of determining platinum in fluor-phlogopite specimens heated in a platinum vessel.Standard samples are prepared by adding a certain amount of platinum to fluor-phlogopite which is free from platinum. As internal standards, Au, Ru and Os are examined by the moving plate method, and Ru is found suitable for the analysis.The sample is mixed with an equal amount of carbon powder which contained 200 ppm of Ru.D.C.arc discharge is used for the excitation.The line pair of Pt 3064.7A/Ru 2988.9 A is used and each line intensity is corrected by background effect.A working curve is obtained for the range between 5 and 500 ppm. Standard deviation in ten times repetition of excitation at each concentration is 10-45 per cent.Satisfactory results are obtained from actual fluor-phlogopite specimens.