A visible-light-induced reaction of an O 3 −trimethylamine (TMA) complex isolated in low-temperature noble-gas matrices is investigated by infrared (IR) and visible absorption spectroscopies using the DFT calculation. The complex isolated in a Ne matrix yields trimethylamine-N-oxide (TMAO) upon irradiation (λ ≥ 800 nm) by dissociation of O 3 . When the wavelength of radiation is changed to λ = 455 nm, two stable conformers of dimethylaminomethanol (DMAM) are recognized besides TMAO. In an Ar matrix, DMAM and not TMAO is mainly produced upon λ = 455 nm irradiation. The photoreaction mechanism of the O 3 − TMA complex with a single collision reaction between O( 3 P) and TMA in the gas phase to produce OH and CH 2 N(CH 3 ) 2 radicals is discussed.