Electroabsorption (Stark effect) spectroscopy using dual-phase lock-in detection was applied to the B850 absorption band of the peripheral antenna (LH2) complex from the purple non-sulfur bacterium Rhodobacter sphaeroides strain G1C. Electroabsorption signals due to the electrostatic interaction between pigments and surrounding apoproteins were clearly detected from the phase-retarded signal. The temperature and frequency dependence of the phase-retarded signal were used to gain insights on the dynamics of the pigment-protein interactions.