The activity of Photosystelil II reaction centers increases in the presence of dichlorobenzoquinone (DCBQ) compared to ferricyanide or dimethylbenzoquinone (DMBQ). We demonstrate that enhanced yields from Photosystem II in the presence of DCBQ depend on flash length (temporal distribution). A broad flash with extended decay enhances oxygen yields while a narrow flash without extended decay (300 ns laser flash) does not. The broad flash causes double turnovers of Photosystem II; the narrow flash does not. Double hlrnovers of Photosystem II oxygen evolution occur in t ~ 5 p.s specifically in the presence of DCBQ and do not contribute to increased electrochromic absorption change. The maximal enhancement by DCBQ occurs at pH 6.5 and a DCBQ-to-chlorophyll ratio of 1: 1. TIle phase of oscillations in 140 p.s delayed fluorescence is advanced by DCBQ as expected of double hIrnovers in Photosystem II reaction centers. We conclude that DCBQ is not accepting electrons from normally inactive Photosystem II reaction centers but is facilitating double hlrnovers of normally active reaction centers. If care is taken to avoid double hlrnovers, the concentration of Photosystem II reactions centers active in oxygen evolution is 1.9 mmol/mol chlorophyll in peas.