The (v', v") bands of the C-1 Sigma(1)-X(1)Sigma(+) transition of YI with v' = 0-7 and v" = 0-4 between 769.2-843.7 nm have been studied using the technique of laser vaporization/reaction with supersonic cooling and laser induced fluorescence spectroscopy. Sixteen vibronic transition bands have been observed and analyzed. A merged least squares fit of the measured line positions yielded molecular constants for both the C(1)Sigma(+) and X(1)Sigma(+) states. The equilibrium bond length of the C(1)Sigma(+) and X(1)Sigma(+) states has been determined to be 2.8280(2) and 2.7637(3) Angstrom, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.