In a process for the manufacture of butynediol by the reaction of formaldehyde and acetylene in aqueous solution, in which a catalyst is used which is produced by the action of acetylene and formaldehyde on copper (II) compounds, the catalyst becomes inactive when it is treated for a relatively long period with formaldehyde at elevated temperature in the absence of acetylene so that it must constantly remain in contact with acetylene-containing solution if its temperature is above 70.degree. C.