The hydrofluoroether C 4F 9OCH 3 (methoxynonafluorobutane, HFE-7100) has been studied by chirped pulse Fourier transform microwave spectroscopy as vapor from the liquid participates in a supersonic expansion of argon. Two isomers are present, (CF 3) 2CFCF 2OCH 3 and n-C 4F 9OCH 3, and in each case the rotational spectra of only one, dominating, conformer has been assigned. Rotational constants, centrifugal distortion constants, and barriers to methyl group internal rotation for the observed species have been experimentally determined for the first time. We note that Rays asymmetry parameter for the (CF 3) 2CFCF 2OCH 3 isomer is 0.007 083(1), indicating almost "perfect" asymmetry. Also, electronic structure calculations show an extremely short C(frame)-O ether bond length of 1.337 Å