The present work deals with the study of the role of vanadium species in the V/Ti-supported oxide catalysts for the oxidation of formaldehyde to formic acid. Two different series of catalysts were prepared, the first one consisting of catalysts of composition 20 wt.% V 2 O 5 /80 wt.% TiO 2 calcined at 400 and 450 β’ C (series 1), and the second one was prepared by washing of series 1 samples with nitric acid (series 2). All the samples were characterized by XRD, IR spectroscopy, temperature-programmed reduction with H 2 , and differential dissolution methods. In the catalysts of series 1, vanadium is represented by both monolayer species (monomeric and polymeric VO x ) and V 2 O 5 phase in crystalline or X-ray amorphous state, depending on the calcination temperature. Washing the samples of series 1 with nitric acid removes the V 2 O 5 phase, and the samples of series 2 contain only monolayer vanadium species. The monolayer vanadia-titania catalysts are highly active in the formaldehyde oxidation to formic acid, while crystalline and amorphous V 2 O 5 exhibit a low activity in that reaction. Moreover, the presence of the V 2 O 5 phase blocks the catalyst active sites and thus decreases the catalyst activity. The monolayer vanadium species are easier to be reduced by H 2 as compared to bulk V 2 O 5 .