Absorption and emission spectra of perylene-3,4-dicarboximide (PMI) and perylene-3,4,9,10-tetracarboxdiimide (PDI) derivatives embedded ill a thin polymer film were measured by room-ternperature bulk and low-temperature single-rnolecule spectroscopy. In contrast to bulk line narrowing spectra, the low-temperature single-molecule data allowed to unambiguously resolve the vibrational fine structure of the emission spectra. Additionally, the emission spectra were calculated by quantum chemical methods within the Franck-Condon approximation for various N-Substituted derivatives of PMI and PDI. The experimental as well as calculated emission spectra ire dominated by two spectral regions of high vibronic activity, a band system ranging from the 0-0 transition (at Delta E0-0) down to 600 cm(-1) below Delta E0-0 and a band system between approximately 1250 and 1700 cm(-1) below Delta E0-0. Apart from the wavenumber region close to Delta E0-0 (down to 100 cm(-1) below Delta E-0-0), good agreement is found between the calculated and experimental spectra, allowing a clear-cut assignment of the dominant vibrational modes. There are, however, discrepancies ill the intensities in particular for low-frequency vibrational modes. These differences between theory and experiment are tentatively attributed to linear electron-phonon Coupling Which is completely neglected in the calculations and hindered internal rotation that is not properly accounted for ill the harmonic approximation. Furthermore, in the experimental spectra, at the bulk as well as the single-molecule level, significant differences between PMI and PDI are observed which are attributed to stronger interactions with the matrix environment in the case Of PMI Clue to the permanent electric dipole moment of that molecule.