Abstract: Using a new type of alkali flame ionization detector (AFID) for the determination of trace amounts of DBF and MBP in the system of 30% -kerosene, two procedures are given:(1) Direct determination of DBF and MBP: in the range of 1X10~B to 5×10-4M. The effect of interfering materials has been investigated. A quan-tatitive method of "three sample injections" is proposed to eleminate the interference of unknown peak (2) The separate determination of trace amounts, of DBP and MBP can be measured, in the range 2×10-7 to 1×10-6M.The separation conditions and results are given. The operation of AFID is described, and a simplified methylating procedure is presented.