From the infrared spectra of the vapor measured between 4000 and 200 cm−1, 2-cyanoethanol was found to exist as gauche and trans isomers in roughly equal proportions around 100 °C. The gauche form is the more stable of the two, as in 2-chloro- and 2-bromo-ethanol, but the enthalpy difference is less than half as large; at most 0.7 kcal mole−1. This indicates some weak (about 1 kcal mole−1) intramolecular hydrogen bonding between the OH group and the π electrons of the C≡N bond.In the solid, two distinct crystalline phases have been observed. Crystal II, consisting of gauche molecules only, is normally obtained; for instance by annealing the vitreous solid from low temperature deposition of the vapor. Crystal I, which contains the two isomers, may be obtained irreversibly from the melt of crystal II under controlled conditions.On the other hand there was no indication in the spectra of a cyclic tautomer as suggested recently.