Vanadium haloperoxidases are found in many algae, lichens, fungi and bacteria, and they also have practical applications in the biotechnology industry. In Nature, they have a crucial role in biosynthetic pathways that lead to antibiotics as well as the emission of greenhouse gases (i.e., chloroform and bromoform) into the atmosphere. Within this class of enzymes, we have focused on vanadium chloroperoxidase (VCPO), which catalyzes the two-electron oxidation of halide to hypohalous acid (Scheme 1); the hypohalous acid diffuses out of the active site and reacts in a nonspecific way with electrophilic substrates. In the fungus, Curvularia inequalis, the synthesized hypohalous acid breaks down the cell membrane of plants which the fungus is attacking. In order to begin the study of the catalytic mechanism of this enzyme, one first has to decide what is its resting state. In many cases, the investigation of the mechanism of catalysis of enzymes requires a determination of the resting state. Scheme 1. VCPO catalyzes the two-electron oxidation of halide.