Photoionization spectra of the titanium and zirconium metallocarbohedrene clusters (Met-Cars) of the stoichiometry Ti8-xZrxC12 (x = 0-4, 8) have been investigated near threshold. Study of the photoionization efficiencies of these species at different photon energies led to a determination that the ionization potentials (IP＇s) for the titanium and zirconium Met-Cars, Ti8C12 and Zr8C12, are 4.40 +/- 0.02 and 3.95 +/-0.02 eV, respectively. The IP＇s for the binary metal Met-Cars, Ti8-xZrxC12 (x = 1-4), were also determined, and it was found that the IP decreases continuously from that of the pure titanium Met-Car toward that of the pure zirconium Met-Car as the number of substituting zirconium atoms increases.