The exciton structure of crystalline PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N-N'-dimethylperylene-3,4,9, 10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes interactions between Frenkel excitons with several vibronic levels and charge transfer excitons. With appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the CT transition dipole, which has a direction different from the Frenkel transition dipole.