This is the first detailed study on the morphology of the widely used particulate electron paramagnetic resonance (EPR) probes lithium phthalocyanine (LiPc), methyl-LiPc, methoxy-LiPc, fusinite and synthetic carbon-based chars, by means of both light and electronmicroscopy (LM and EM). The importance of these EPR probes for the measurement of O2/NO has been reported previously. Under LM, LiPc with its distinct crystalline structure differs significantly from the noncrystalline black flakes of both LiPc derivatives, methyl-LiPc and methoxy-LiPc. Unlike the shiny carbon-based synthetic chars, which have no characteristic morphology, fusinite, a fraction of fossilized coal maceral, displays distinctive fine, parallel channels. SEM studies reveal a striated surface and interlocking multilayered structure of LiPc that is markedly different from either the multilayered stacked methyl-LiPc or botryoidal methoxy-LiPc. The regularly spaced pores and channels of fusinite, a reflection of its plant origin, contrast sharply with the randomly distributed pores of all sizes of the various synthetic chars. Furthermore, the combined results of both LM and EM studies strongly suggest the nonperturbing nature of fusinite and LiPc when they were used as EPR probes to measure oxygen in cells or tissues. We hope that this study, in conjunction with the numerous EPR functional studies of probes reported earlier, provides valuable information for the future development of new particulate EPR spin probes.