The details of weak C-H… p interactions that control several inter and intramolecular structures have been studied experi- mentally and theoretically for the 1:1 C2H2-CHCl3 adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C2H2 and CHCl3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-3111G(d,p) and B3LYP/6-3111G(d,p) levels. Both the computational and experimental data indicate that the C 2H2-CHCl3 complex has a weak hydrogen bond involving a C-H… p interaction, where the C2H2 acts as a proton acceptor and the CHCl3 as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl3 and a hydrogen in C2H2. The combination of the C-H… p interaction and the secondary Cl… H interaction determines the structure and the energetics of the C2H2-CHCl3 complex. In addition to the vibrational assignments for the C 2H2-CHCl3 complex we have also observed and assigned features owing to the proton accepting C2H2 submolecule in the acetylene dimer. q 1999 Elsevier Science B.V. All rights reserved.