Acid- or base-catalyzed condensations of carbonyl compounds, hydrolyses, eliminations, ring-opening reactions, rearrangements, oxidations, reductions, etc. are sometimes reported to occur during column chromatography on alumina or silica gel. These reactions are classified according to the type of the reactions and reviewed. In recent years reactions of reactive substrates (e. g., epoxides) with mild reagents (e. g., H₂O, R₃N) are carried out in the presence of alumina or silica gel and products distributions are found to be changed appreciably. These reactions are also included here.