Rapid-scan FTIR difference spectroscopy was used to investigate light-induced reduction of the ubiquinone QB, and the oxidation kinetics of QB and QH2 by external mediators in isolated reaction centers (RCs) from R. sphaeroides. As redox mediators, a Ferrocyanide/N,N,Nʹ,Nʹ-tetramethyl-p-phenylenediamine (TMPD) mixture and an Ascorbate/2,3,5,6-tetramethyl-p-phenylenediamine (DAD) mixture are compared.Results show that TMPDred rapidly reduces the photoproduced P870+ primary donor. The process is fast enough to record rapid-scan FTIR spectra devoid of P870+ bands down to 260 K. Results show also that TMPDox oxidises both QB and QH2 faster than DADox. In particular, QB is oxidised faster than QH2 at all temperatures studied.Results are discussed in the framework of time-resolved infrared studies on R. sphaeroides RCs, showing advantages/drawbacks of the proposed experimental approach.