Detailed studies of infrared absorption in nominally undoped and boron‐doped, free‐standing diamond thin films are reported. Difference measurements reveal absorption at 1300 cm−1(0.16 eV) due to boron‐induced single phonon, vibronic excitations. A relatively sharp peak at about 2420 cm−1(0.30 eV), a stronger, broader band centered at 3060 cm−1(0.38 eV), and a weak, broad peak at 4200 cm−1(0.52 eV), are identified as electronic transitions, with or without phonon assistance, of the boron acceptor. These results provide important confirmation of the hitherto presumed substitutional nature of boron doping and recent suggestions concerning electronic transport mechanisms in diamond thin films.