Two different axial and equatorial hydrogen-bonded conformers of the complex formed by pentamethylene sulfide and hydrogen fluoride have been generated in a pulsed supersonic expansion and characterised by means of Fourier transform microwave spectroscopy. The ground-state rotational spectra of six isotopomers (C(5)H(10)S...HF, C(5)H(10)S ...DF, C(5)H(10)(34)S ...HF, (13)C(alpha)C(4)H(10)S ...HF, (13)C(beta)C(4)H(10)S...HF and (13)C(gamma)C(4)H(10)S ...HF) have been analysed for both conformers in the frequency range 5.5-18.5 GHz. The rotational parameters were used to derive C(s) structures for the conformers, with hydrogen fluoride pointing to the domain of the nonbonding electron pairs at either the axial or equatorial position of the sulfur atom. The axial form was found to be the more stable, in contrast with the observation for the pentamethylene sulfide...HCl complex. No equatorial-to-axial relaxation was observed when He or Ar were used as the carrier gas. The conformational behaviour is compared with that of related six-membered rings and discussed in terms of the existence of secondary hydrogen bonding between the halogen atom and the nearest H atoms of the methylene groups of the ring. No significant structural distortion of pentamethylene sulfide upon complexation was detected from a comparison with the structure of the isolated monomer. Finally, an ab initio study was carried out to complement the experimental results.