The Fourier transform microwave investigation of C6H6-DCF3 outlines a shortening of the distance between the two constituent molecules of about 0.0044(2) Å upon H → D substitution of the hydrogen atom involved in the C-H···π hydrogen bond. This proves that the Ubbelohde effect takes place also within weak hydrogen bonding. The measure of the spectra of several (13)C isotopologues in natural abundance has been useful to obtain structural information.