Molybdenum oxynitrides are very sensitive and attention should be paid to their storage and pretreatment in order to get accurate and reproducible physisorption or chemisorption results, This work showed that storage of these compounds in air leads to loss of accessibility of the surface area, illustrated by an apparent decrease of the specific surface area and pore volume. This phenomenon is due to the high affinity of these materials for oxygen, which diffuses into their lattice. Furthermore, oxygen, ammonia, and water, present on the surface of molybdenum oxynitrides after preparation and passivation, can block the access of probe molecules to pores or active sites. A method of storage and pretreatment before physisorption or chemisorption experiments is presented. CO chemisorption experiments in static and dynamic pulsed conditions were performed and led to similar results as in experiments performed with samples without any residual oxygen contamination. Low CO uptakes were interpreted in terms of nonstoichiometry of the compounds.