Infrared spectra of 1,2-dimethylhydrazine in low-temperature rare-gas matrices were measured with an FTIR spectrophotometer. The observed bands were assigned to two stable isomers, outer-outer and inner-outer, being aided by density-functional-theory calculations. Spectral changes caused by the isomerization from inner-outer to outer-outer were observed when the matrix sample was kept in the infrared radiation of the spectrophotometer, or even in dark without infrared radiation. It is concluded that the isomerization occurs by inversion due to tunneling of hydrogen-atoms and that outer-outer isomer is slightly more stable than inner-outer. (C) 2004 Elsevier B.V. All rights reserved.