This paper investigates the existence of side reactions involving oxygen donors in the epoxidation of cyclohexene in the presence of iron and cobalt phthalocyanines. An evaluation of the catalytic role of fragments coming from the degradation of phthalocyanines in the overall reactions has been undertaken. It is shown that neither tert-butyl-hydroperoxide (t-BHP) nor iodosylbenzene (PhIO), two commonly used donors, are convenient for evaluating the catalytic performances of phthalocyanines in the epoxidation of olefins. With t-BHP, the oxidation reaction is, at least partially, violently triggered by fragments from phthalocyanine decomposition; the reaction involves homogeneous radical pathways, and mainly leads to oxidized derivatives different than the epoxide. Its use necessitates precautions to preclude this decomposition. In the case of PhIO, the oxygen donor oxidizes the substrate even in the absence of catalyst. By contrast, hydrogen peroxide does not give way to major side reaction; it is efficient and selective for the heterogeneously catalyzed epoxidation of cyclohexene. It is therefore suitable, without special procedure, for studying the activity of phthalocyanines. Due to some spontaneous decomposition of H,O,, some precautions are nevertheless necessary to evaluate correctly the kinetics of the reaction.