The k(3) Pi state of the CO molecule is investigated in the region between 91 000 and 97 000 cm(-1) via 1+1 resonance enhanced multiphoton ionization spectroscopy on CO molecules prepared in a single quantum level of the a(3) Pi(upsilon = 1) state. A new vibronic band is found which is at lower energy than the vibrational ground state reported in the literature, leading to a reassignment of the vibrational numbering of the k(3) Pi state, The rotationally resolved spectra of the k(3) Pi (upsilon = 0-6) <--a(3) Pi(upsilon = 1, J = 1, Omega = 1) of (CO)-C-12 and (CO)-C-13 have been observed and analyzed, confirming the new vibrational labeling and providing a full set of molecular constants of the k(3) Pi valence state. (C) 1997 American Institute of Physics. [S0021-9606(97)02644-5].