The oxirane-trifluoromethane dimer generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. The rotational spectra of the parent species and of its two 13C isotopomers in combination with ab initio calculations have been used to establish a Cs geometry for the dimer with the two monomers bound by one C-HâââO and two C-HâââF-C hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-HâââO and C-HâââF hydrogen bonds, r(OâââH) and r (FâââH), are 2.37 and 2.68 Å, respectively. The C-HâââF-C interactions give rise to the HCF3 internal rotation motion barrier of 0.55(1) kJ/mol, which causes the A-E splittings observed in the rotational spectra. The analysis of the structural and energetic features of the C-HâââO and C-HâââF-C interactions allows us to classify them as weak hydrogen bonds. Ab initio calculations predict these weak interactions to produce blue shifts in the C-H vibrational frequencies and shortenings of the C-H lengths.