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