MCSCF and MR-ACPF calculations on the structure and vibrational frequencies of allyl (2-propenyl) and 1,4-pentadienyl radicals are presented. Using different levels of theory the instability problem in allyl has been investigated in great detail. Based on these conclusions fully optimized structures and force constants were calculated at the three-orbital CAS-MCSCF and at the MR-ACPF level for allyl. To correct for systematic errors, an empirical scaling was also applied on the MCSCF force field. The scaled MCSCF and the unadjusted MR-ACPF force constants were used to calculate the vibrational frequencies. A reassignation of the experimental infrared spectrum is also given. Using the scale factors optimized for allyl the scaled MCSCF force field and vibrational frequencies of pentadienyl were also calculated.