Sub-Doppler spectra of the A?3?–X?3S-(0,0) and (1,0) bands of the PH radical have been recorded using an injection seeded single mode optical parametric oscillator in a supersonic jet expansion. Most of the rotational lines in these laser-induced fluorescence spectra exhibit clear splittings or asymmetry due to hyperfine structure. An analysis of this structure is presented in terms of the electronic structure and bonding of the molecule. Comparisons are drawn with the corresponding A?3? state of the NH radical, and some shortcomings in the accepted methods for interpretation are highlighted and discussed. © 2003 American Institute of Physics.