Naphthacene and fluoranthene in hexane showed photoconductivity signals upon excitation by 458-nm irradiation (200 mW) with a continuous-wave argon ion laser. The signals were quadratically proportional to the laser power and linearly proportional to the solute concentration, indicating a two-photon excitation-ionization. By evaluation of the signal intensity and laser beam parameters, two-photon ionization cross sections for the production of the free ions separating from geminate ion-electron pairs have been determined absolutely to be 2.2 X 10(-52) and 2.5 X 10(-54) cm4 S/(molecule photon) for naphthacene and fluoranthene, respectively, at an electric field of 5000 V/cm. The former cross section of the ionization via a real state was about 100 times larger than the latter via a virtual state. The calculated quantum yield of naphthacene from the two-photon ionization cross section is smaller by about 6 orders of magnitude than the reported value of anthracene obtained by the pulsed laser excitation.