Methanol can be converted to pure, and in particular water-free, formaldehyde catalysed by compounds containing sodium as potentially active species. A new way of supplying the catalyst was examined. Elemental sodium was vapourized and mixed with methanol in the vapour phase yielding pure formaldehyde and hydrogen. The degree of conversion was higher than 95%, the selectivity of formaldehyde higher than 80% detecting carbon monoxide as by-product. The reaction mechanism can be explained if a homogeneous vapour-phase catalysis is assumed.