A commercial inductively coupled plasma (ICP) mass spectrometer was modified to employ an air-hydrogen flame in place of the ICP as an ion source. A liquid nitrogen trap was placed in the vacuum line to remove water. A very simple intrinsic mass spectral background was obtained with the hydrogen flame ionization mass spectrometry (FIMS). Molecular ions such as K(H ₂ O) ⁺ , Na(H ₂ O) ⁺ , Ca(H ₂ O) ⁺ and CaOH(H ₂ O) _x ⁺ (x=0-2) were observed when solutions containing Na, K or Ca were aspirated. Although the presence of the molecular ions complicated the mass spectra, it also provided a wider choice of analytical masses for an analyte. Isotope ratio measurements of Ca were made with both Ca ⁺ and CaOH ⁺ species at masses 40, 44, 57 and 61. Better isotope ratio precision was obtained at CaOH ⁺ masses relative to those for Ca ⁺ because the sensitivity was about 10 times higher. Isotope ratio measurement of K was made at masses 39 and 41. A ratio precision of about 0.2 and 0.5% was obtained for K and Ca, respectively. The results suggest that the FIMS is suitable for the isotope ratio measurement of K and Ca in simple matrices, and that the air-hydrogen flame is a more desirable ion source than an air-acetylene flame for FIMS.