Electroabsorption (Stark effect) spectroscopy using dual-phase lock-in detection was applied to the B850absorption band of the peripheral antenna (LH2) complex from the purple non-sulfur bacterium Rhodobactersphaeroides strain G1C. Electroabsorption signals due to the electrostatic interaction between pigments andsurrounding apoproteins were clearly detected from the phase-retarded signal. The temperature and frequencydependence of the phase-retarded signal were used to gain insights on the dynamics of the pigment−proteininteractions.