Chlorite dismutases (Clds) are metalloproteins containing heme b (i.e. Fe(III) in a porphyrin system) as redox cofactor. Clds are able to degrade toxic chlorite to chloride and dioxygen. This reaction is interesting from a biotechnological point of view as chlorite is a serious environmental pollutant and its bioremediation is an issue. However, mechanism of catalysis is not fully understood yet. Chlorite degradation activity is efficient at acidic pH, exhibiting its optimum at pH 5.0, but decreases significantly with rising pH. It is hypothesized that this depends, at least to a certain extent, on the protonation state of a conserved arginine residue. Investigating the protonation state of this residue under certain pH conditions by using neutron crystallography will highly contribute to understand Clds' reaction mechanism in general and the role of the conserved arginine in particular.