It is shown that step-scan Fourier transform infrared spectroscopy can be applied to resolve the QA-QB → QAQB- transition in Rhodobacter sphaeroides reaction centres with a 5 µs time resolution. In the mid-infrared region (1900 – 1200 cm-1), transient signals previously assigned to QA/B and QA/B- vibrations, respectively (Brudler et al. 1994; Brudler et al. 1995; Breton and Nabedryk 1996), can be resolved with this new technique. In addition, the three small positive bands in the spectral region of the carboxylic C=O stretching modes of acidic amino acid side chains are also resolved at 1730, 1719 and 1704 cm-1. A global fit analysis yields two exponentials with half-times of 150 µs and 1.2 ms in agreement with IR spectroscopic studies at single wavenumbers (Hienerwadel et al. 1995), in the UV/VIS and near IR (Tiede et al. 1996, Li et al. 1996). The establishement of the step-scan technique enables a new approach to elucidate the molecular mechanism of this transition.