Crotonaldehyde is an interesting intermediate in the chemical industry. It is usually produced from aqueous acetaldehyde in a two step process in which the first step is carried out under basic and the second step under acidic conditions. It is commonly assumed that acetaldehyde is converted in the first step to acetaldol and that acetaldol is subsequently dehydrated in the second step to crotonaldehyde. We demonstrate by H-1 and C-13 NMR spectroscopic studies that acetaldol is hardly present in the reacting solutions at lower temperatures and that the key intermediate is aldoxane (2,6-dimethyl-1,3-dioxane-4-ol). For the first time, data on the chemical equilibrium of the aldoxane formation in aqueous acetaldehyde solutions is provided. Furthermore, preliminary information on the kinetics of that reaction is presented.