We use infrared absorption spectroscopy to study both the electronic excitation and the local vibrational mode (LVM) of the carbon acceptor in semi‐insulating GaAs as a function of EL2 photoquenching. The behavior of the far‐infrared electronic lines shows that the normally compensated carbon becomes neutralized (i.e., changes its charge state from singly negative to neutral) when the EL2 absorption is photoquenched. At the same time, the LVM absorption band shows no change in spectral form or vibrational frequency of its fine structure after photoquenching which indicates that, contrary to expectations, a charge‐state change has no measurable effect on this band.