Polymerization of acetone and acetylene, the decomposition products of methylbutynol (MBOH), on the surface of Y/MgO catalyst was inferred using high-temperature transmission FTIR spectroscopy. It was found that MBOH decomposes into acetone and acetylene revealing the basic surface character of Y/MgO catalyst. However, modification of MgO with Y3+ cations decreased the catalyst activity towards MBOH decomposition relative to that observed for pure MgO. The consecutive polymerization of the decomposition products is responsible for this low activity, since polymerized acetone is strongly adsorbed on the catalyst surface, and hence, blocks the surface active sites.