A technique for determining the ionization potentials of pure liquid samples using visible wavelength femtosecond pulses is described. Exploiting a multiphoton ionization mechanism, we present values for the liquid-phase ionization potentials and compare them with estimated values calculated from gas phase ionization potentials. Estimates rely upon theoretical calculations of two values. First, the polarization energy of the solvent in the presence of a charged solute is calculated using the Born continuum solvation (BCS) model. Second, the conduction band of the liquid sample is calculated using the Springett - Jortner - Cohen (SJC) model, which corresponds to the stabilization energy of the free electron in the dielectric medium. Polarization energies are calculated with two different estimates of cavity radii, and comparisons are made to determine the adequacy of each set of radii. The striking agreement between the experimental results and theoretical predictions supports the viability of this technique as a straightforward method for determining liquid-phase ionization potentials using visible wavelength photons. It also supports the BCS model as an applicable method for calculating solvent polarization energies suitable for femtosecond photoionization.