A process for preparing compounds of the formulae I and II ##STR1## where R.sup.1 and R have the meanings indicated in the specification by an addition reaction of a compound of the formula IIIR.sup.1 OH IIIwith an acetylene or allene of the formula IV or V ##STR2## in the gas phase at elevated temperatures in the presence of a heterogeneous catalyst. The catalyst is obtained by impregnating silica with a zinc salt, by a process in which the reaction is carried out at below 200.degree. C. and the catalyst contains, as an active component, an X-ray amorphous zinc silicate or cadmium silicate containing from 1 to 40% by weight, calculated as oxide, of zinc or cadmium, obtainable by applying a salt of zinc or cadmium and an inorganic oxo acid, which salt is decomposable at below 400.degree. C., to amorphous silica and forming the catalyst before the reaction at from 50 to 500.degree. C. or during the reaction in situ at from 50 to 200.degree. C. in the presence of a hydroxyl-containing compound such as water, alkanols of 1 to 6 carbon atoms, diols and polyols having 2 to 6 carbon atoms and 2 or 3 OH groups and low molecular weight carboxylic acids.