4,5-Dihydro-2-methylfuran (DHMF) can be formed from cyclization of 5-hydroxy-2-pentanone, an important gas-phase photooxidation product of n-pentane and a representative 1,4-hydroxycarbonyl. At very low (much less than1%) relative humidity a lifetime of similar to1.1 h was obtained for the transformation of 5-hydroxy-2-pentanone to DHMF. Rate constants and products of the gas-phase reactions of DHMF with OH radicals, NO3 radicals, and ozone have been determined at 298 +/- 2 K and atmospheric pressure of air using in situ Fourier transform infrared (FT-IR) spectroscopy, in situ atmospheric pressure ionization tandem mass spectrometry (API-MS/MS), and combined gas chromatography-mass spectrometry (GC MS). Rate constants (in cm(3) molecule(-1) s(-1)) for the reactions of DHMF with OH radicals, NO3 radical's, and O-3 were measured to be (2.18 +/-0.11) x 10(-10), (1.68 +/- 0.12) x 10(-10), and (3.49 +/- 0.24) x 10(-15), respectively, resulting in estimated tropospheric lifetimes of 1.3 h, 24 s, and 7 min for the OH radical, NO3 radical, and O-3 reactions, respectively. The yields of identifiable products from the atmospheric reactions of DHMF were quantified, and possible mechanisms for their formation are discussed.