The 444 nm 2(0)(1) bands of the (A) over tilde (2)A'- (X) over tilde (2)A" transition of the jet-cooled HP Cl-35 and HP Cl-37 radicals have been studied at high resolution using the pulsed electric discharge R technique with a precursor mixture of PCl3 and H-2. Spectra recorded with linewidths of similar to360 MHz revealed resolved hyperfine structure in both isotopomers arising from the excited state Fermi contact interaction of the unpaired electron with the magnetic moment of the P-31 nucleus, with a(F)' = 0.0641(10) cm(-1) and 0.0636(31) cm(-1) for HP Cl-35 and HP Cl-37, respectively. No contribution from the ground state, or excited state contributions from the hydrogen or chlorine nuclei were resolved, confirming ab initio predictions that HPCl is a ppi radical in the (X) over tilde state, and an s sigma radical with a substantial contribution from the phosphorus 3 s atomic orbital in the (A) over tilde state. The free atom comparison method has been used to estimate that the singly occupied molecular orbital in the excited state has 14% phosphorus 3s, character. (C) 2004 American Institute of Physics.