The spectroscopic examination in the infrared of the gases existing after the contact of acetaldehyde with a silica alumina catalyst at 200°C, for gradually increasing times, shows the gradual disappearance of the bands referable to crotonaldehyde. That indicates that the crotonaldehyde, developed in the catalyst owing to the condensation of the two acetaldehyde molecules, is desorbed after its formation. At the same time we can observe a general decrease of the spectra intensity, and most probably we can refer that to the dark gummy product formation on the catalyst, at the expense of the present gases.