Although the alkaline earth–monohalide compounds, MX, have been extensively studied, their ionization potentials to MX+ are only known to within a few tenths of an eV. Using an improved electron impact ionization potential measurement to assist in arranging spectroscopic vibronic band origins into Rydberg series, we report high precision ionization potential measurements for CaF and BaF to be IP(CaF)=5.8270±0.0006 eV and IP(BaF)=4.8034±0.0004 eV. The synergism between the electron impact method and the spectroscopic method is detailed, and the minimum accuracy required of the electron impact data is assessed.