Several scenarios were evaluated to explain the large "dendrimer effect" observed in the bromination of cyclohexene with H2O2 and NaBr catalyzed by the addition of Frechet-type dendrimers terminating in -O(CH2)(3)SePh groups. Although phenylseleninic acid was an efficient catalyst for the oxidation of NaBr with H2O2, first-order rate constants for the selenoxide elimination were too small to produce PhSeO2H at a rate sufficient to explain the rates of catalysis and no dendrimer effect was observed in the rates of selenoxide elimination. An induction period was observed using 1-SePh as a catalyst for the oxidation of Br- with H2O2. The addition of preformed selenoxide 1-Se(=O)Ph gave immediate catalysis with no induction period. However, rates of oxidation of the selenides with H2O2 under homogeneous or biphasic conditions or with t-BuOOH under homogeneous conditions were too slow to account for the rates of catalysis, and no dendrimer effect was observed in the rates of oxidation. The primary oxidant for converting selenides to selenoxides was "Br+" produced initially by the uncatalyzed background reaction of H2O2 with NaBr and then produced catalytically following formation of selenoxide groups. Autocatalysis is observed, and the rate of oxidation increases with the number of SePh groups. Autocatalysis is the source of the large dendrimer effect observed with the SePh series of catalysts.