Pulsed zero-field electron paramagnetic resonance free induction decay (ZF EPR FID) techniques are employed to study the phase transition of p-terphenyl crystals (T-c = 193 K) by measuring selectively populated photoexcited triplet ZF transitions of dilute pentacene molecules in p-terphenyl single crystals. The sensitivity of ZF spectroscopy to small shifts in local magnetic fields enables the studies of guest-host configuration changes over a wide temperature range. Here we report the observation of guest pentacene (-h(14) and -d(14)) triplet ZF EPR FID spectra that disappear abruptly at T-c and of spectral broadening and shift below T-c. We interpret these spectral changes as evidence for guest couplings to host phenyl rings. Further, these data allow assignments of spectroscopic sites to crystallographic sites that occur in the phase transition.