The determination of small amounts of potassium ions in soils requires sensitive and precise chemical methods. Measurements of potassium and sodium by the chemical methods are time consuming. The flame photometer method of measuring these ions is more rapid and has proved reliable for soils under temperate environmental conditions. It was the purpose of this study to determine whether the rapid flame photometer method of measurement of exchangeable potassium and sodium could be used effectively and with .accuracy on soils in São Paulo. The first results, in the experiments undertaken, were obtained using the Perkin-Elmer flame photometer Model 18 ; subsequently the newer model 52-A was employed. The data obtained from the tests have; been found to give a general indication for determining exchangeable potassium in the soils of the State of São Paulo. The following method of treatment of soil samples was found to give the most reliable results. After extraction of the cations with a solution of HNO3 0.05 N, the soil solution was neutralized with NH4OH. This solution was then placed in the photometer in order to measure the amount of potassium by means of reading directly the intensity of radiations emitted by this element. Ultragas (propane) mixed with air and at a preassure of 6 to 7 pounds per square inch was used as a source of energy. The apparatus used in these tests permitted determination of potassium in solution from 0.00001 N of K+ (0.39 ppm of K) to 0.001 N (39 ppm of K). Within these limites the readings were proportional to the concentrations and can be represented by a straight line. A study was also made of the interference of the elements Ca ++, Mg++, Al +++ and Fe +++ and it was found that these ions, in the concentration in which they usually exist in the soil solutions, did not interfere with the determination of potassium. In order to verify the accuracy of the determinations of potassium by the flame photometer, the method of cobalti-hexanitrite was employed using comparable soil samples. In general the results obtained by both methods were? the same, thus confirming the accuracy and sensitivity of the flame photometer. This latter method has a great advantage in that it is possible to make 50 to 00 determinations of potassium in one day whereas the cobalti-hexanitrite method is more laborious and allows analyses of only about 8 samples per day.