The alkali flame ionization detector (AFID) is used nearly exclusively for the selective gas chromatographic determination of P- and N-containing pesticides. In some respects this detector exceeds the electron capture detector (ECD). We have shown that the estrone dimethylphosphinic ester has good gas chromatographic properties and can be measured very sensitively with the AFID. A modified procedure of derivatization is described. The monohydroxy steroid is treated with dimethylaminodimethylphosphine and sufficient oxygen for 0.5 to 2 hr at 50 to 70°C. After a thin layer chromatographic (TLC) purification step, the derivative is determined with a gas chromatograph equipped with a Rb2SO4 detector. The phosphinylation reaction has been extended to a row of monohydroxy substituted steroids. The influence of steric hindrance on the reaction kinetics of various steroids is discussed. The structure of the steroid derivatives has been confirmed in the case of the estrone ester by spectroscopic and chemical methods. The correlation coefficients of various calibration curves with increasing amounts of the respective steroids were 0.997. The smallest amount that we could measure with good precision was in the range of 10 pg corresponding to 4.5 × 10−14 gP/sec.