A Fourier transform infrared study of Fe‐donor B‐acceptor complexes formed in iron‐implanted B‐doped Czochralski‐grown silicon crystals is performed by monitoring the behavior of boron‐acceptor excitation spectrum lines. The effect of the iron presence on absorption spectra due to B‐acceptor hydrogenlike systems, related to electronic transitions from the ground to excited states associated with the siliconP3/2valence band, was analyzed in a wide range of fluences. Low‐temperature optical data are reported and discussed comparing optical results with secondary‐ion mass spectroscopy and spreading resistance measurements, supporting the existence of a fluence threshold that controls iron diffusion into the bulk.