The deep oxidation of n-hexane was studied over Pt catalysts with small and large Pt crystallites, promoted by MnO x . The Mn was deposited on the Pt/Al 2 O 3 by deposition-precipitation method using two alkalis, ammonia and dimethylamine. The strength of the base influenced morphology and performances of Mn-Pt catalysts. Ammonia formed mostly spherical structure of manganese oxide, while dimethylamine created fibrous needle-like shape of MnO x , typical for cryptomelane phase of manganese oxide. This was confirmed by XPS results that show an oxidation state of Mn close to that for cryptomelane. The Pt-Mn catalysts were more active than catalysts containing Pt only. Promoting effect of MnO x is more pronounced for larger Pt crystallites. Among Pt-Mn samples, catalysts synthesized with dimethylamine exhibited the highest activities. Mobility and reactivity of oxygen from Pt–O–Mn sites associated with cryptomelane phase may be responsible for increased activity.