Abstract The condensation of a number of carbonyl compounds with ethanolamine and with amino‐alcohols having a tertiary hydroxy‐ and a primary amino‐group, has been studied. The molecular refraction and the infrared spectrum have been used to distinguish between the oxazolidine and Schiff's base structures which are both possible for the condensation products. Aromatic and α,β‐unsaturated carbonyl compounds, on the whole, tend to form Schiff's bases, saturated carbonyl compounds oxazolidines, with ethanolamine. Introduction of C‐alkyl groups into the ethanolamine system favours the formation of oxazolidines even with aromatic aldehydes and ketones. In the infrared spectrum of simple CN‐compounds in alcoholic solution, a solvation effect has been observed. Also, the tautomerisability of substances containing the system CH−1—CN, to HC C—NH has been demonstrated. The (non‐conjugated) CN double bond absorbs in the infrared at about 1670 cm−21 whilst the oxazolidine system is characterised by three bands (1149—1185; 1116—1139; 1086—1118 cm−1) in the 1100—1200 cm−1 region. The similarity of the infrared spectra of all heterocyclic substances of the general type magnified image is stressed.