3-phenyl-1,2,5-selenadiazole (I), 3-phenyl-1,2,5-thiadiazole (II), and 3-phenyl-1,2,5-oxadiazole (III) undergo bromination at the para position of the phenyl ring. Compound I undergoes mononitration and also dinitration at the position para and ortho of the phenyl ring in analogy to II and III. Towards either bromination or nitration the more reactive of the examined substrates is compound II which is known to have interatomic ring distances in agreement with a higher delocalization of $\pi$-electrons. The electrophilic substitution never involves the heterocyclic ring, which is still an ortho-para directing group.