The bulky para substituent of bisphenol A (1) restricts rotation around the C4-C7 bond in the phenol, phenoxide, and phenoxyl radical. This hindered rotation is responsible for modulation of the hyperfine interaction between the impaired electron and the meta protons on the phenoxyl ring. The modulation manifests itself in the time-resolved electron paramagnetic resonance (TREPR) spectrum of the bisphenol A phenoxyl radical as selective broadening of certain transitions. Line widths of the broadened transitions of the bisphenoxyl A TREPR spectrum at different temperatures were obtained by simulation of the TREPR spectra, and an energy barrier of 6.2 +/- 1 kcal mol(-1) for this rotation was estimated using a simple two-state model for rotation around the C4-C7 bond.