The different conformations of molecular compounds play important roles in biochemistry and organic solid-state technology. Hydrostatic pressure has been applied to para-quaterphenyl to probe its molecular structure at liquid-helium temperatures. The molecules transform from a twisted to a planar conformation at a critical pressure of 0.9 GPa. This conformational change results in the abrupt disappearance of five infrared-absorption peaks. A group-theoretical analysis shows that the peaks, which correspond to out-of-plane hydrogen bending modes, disappear due to the transition from a low to a high symmetry. (C) 2001 American Institute of Physics.