The fluorescence excitation spectra and the fluorescence decays of phenanthrene-h10 and phenanthrene-d10 in a supersonic free jet have been measured with excitation across the S0→ S1 and S0→ S2 absorption regions. The fluorescence excitation spectra of the isolated phenanthrene molecule are compared with the absorption spectra in low temperature matrices. The presence of a2-type out-of-plane vibrational bands in the S0→ S2 absorption spectra indicates that a-1B1(∏a+) state lies near the S2 state. The S2→S1 internal conversion rates at various vibronic levels of S2 for both compounds have been evaluated from the analysis of profile and width of the individual vibronic bands in the excitation spectra. The excess vibrational energy dependence of the S2 →S1 internal conversion rate is the opposite of that shown by the total decay rate in S1. The excess vibrational energy dependence of the electronic decay rate and the profile of the vibronic absorption bands belonging to the S0 →S2 transition are discussed from the viewpoint of the electronic energy gap.