Compounds of the formulae I and II ##STR1## are prepared 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## where R.sup.1 and R are as defined, in the gas phase at elevated temperatures in the presence of a heterogeneous, silicate-containing catalyst, by a process in which the catalyst used is one which contains or consists of, as the active component, a zinc silicate obtained by precipitation in aqueous solution from a soluble silicon compound and zinc compound, zinc silicate beinga) an essentially X-ray amorphous zinc silicate of the formula VIZn.sub.a Si.sub.c O.sub.a+2c-0.5e (OH).sub.e.fH.sub.2 O VI, where e is from 0 to 2a+4c and the ratio a:c is from 1 to 3.5, and the ratio f:a is from 0 to 200, and/orb) a crystalline zinc silicate having the structure of hemimorphite of the formula Zn.sub.4 Si.sub.2 O.sub.7 (OH).sub.2.H.sub.2 O.