Abstract: When progesterone compounds were mixed with an appropriate of NaCl and KCl, in the high mass region of their fast atom bombardment mass spectra appeared two strong ion peaks M+Na⁺ and M+K⁺. This pair of adductions were easily recognized due to the fact that they were very strong (their abundances were higher than that of corresponding M+H⁺ in several tens fold) and their mass difference was 16 amu and may give exactly the information about molecurlar weight.