Optical absorption and electron paramagnetic resonance (EPR) have been used to study the sites and the charge states associated with Fe ions in single crystals of KH2PO4 (KDP). Doping KDP with 5 ppm of Fe introduced a broad charge-transfer absorption band near 270 nm. At the same time, room-temperature EPR showed that two Fe3+ centers were present. Although their intensities are different, the two EPR centers have similar zero-field splittings. Spin-Hamiltonian parameters, including fourth order in crystal field, were determined for the dominant center. We suggest that these EPR-active Felt ions substitute for KI ions, presumably with charge-compensating cation vacancies (i.e., potassium or hydrogen) nearby. Crystals containing both the 270-nm absorption band and the Fe3+ EPR spectra were X-ray irradiated at room temperature and then subjected to a series of thermal anneals up to 150 degreesC. These treatments caused changes in both the absorption band and the EPR spectra, but the behaviors of the optical and EPR features did not correlate. This can be explained if a portion of the Fe ions occupy a site in the KDP crystals other than the K+ site. We suggest that most of the optical absorption in the 200-to-300nm region is due to Fe ions substituting for phosphorus; i.e., (FeO4)(2-) molecular units are present in the KDP crystals and give rise to ultraviolet absorption.