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Reaction centers (RC) from the species Erythrobacter (Eb.) litoralis, Erythromonas (Em.) ursincola and Sandaracinobacter (S.) sibiricus have been purified by LDAO treatment of light-harvesting-reaction center complexes and DEAE chromatography. The content and overall organisation of the RCs' chromophores, determined by linear dichroism (LD) and absorption spectroscopy, are similar to those isolated from anaerobic photosynthetic bacteria. The redox properties of the primary electron donor are pH-independent and very similar to those determined for anaerobic photosynthetic bacteria with midpoint potential values equal to 445 (± 10), 475 and 510 mV for Eb. litoralis, S. sibiricus and Em. ursincola, respectively. The RC purified from Eb. litoralis does not contain bound cytochrome (cyt), whereas RCs isolated from S. sibiricus and Em. ursincola possess a tetraheme cyt c. Each of these tetraheme cyts contains two high potential hemes and two low potential hemes. Their redox properties are very similar, with midpoint potentials equal to 385 (± 10), 305, 40, -40 mV for Em. ursincola and 355, 285, 30, -48 mV for S. sibiricus. At physiological pH, the midpoint potential of the primary electron acceptor (QA) varies with a slope of -60 mV/pH unit. The reduced form of QA presents pK values of 9, 9.8, 10.5 for S. sibiricus, Em. ursincola and Eb. litoralis, respectively. The main difference observed between RCs isolated from anaerobic photosynthetic and from obligate aerobic bacteria is the Emvalues of QA which are 65 to 120 mV higher in the last case. This difference is proposed to be a major reason for the inability of these species to grow under anaerobic photosynthetic conditions.