Changes in the conformations of conjugated molecules affect the optical and electronic properties significantly. Hydrostatic pressure has been used to probe the conformations of biphenyl (C12H10) and deuterated biphenyl (C12D10) at liquid-helium temperatures. Infrared (IR) spectra of these materials have been taken up to a pressure of 2 GPa. A disappearance of certain IR absorption peaks has been found to occur between 0.07 and 0.45 GPa, due to the phase transition from a twisted to a planar conformation. Numerical simulations together with group-theoretical analysis have been performed to identify the nature of the vibrational modes that lose IR activity upon planarization.