We report on the first detailed studies of the spectroscopy of an iodocarbene, measuring fluorescence excitation and emission spectra of the (X) over tilde (1)A'->(A) over tilde (1)A '' system of :CHI and the deuterated isotopomer :CDI. Due to similar bending and C-I stretching frequencies in the upper state, fluorescence excitation spectra of :CHI show polyads composed of members of the 2(0)(n-x)3(0)(x) progressions with x = 0-3. For :CDI, only progressions with x = 0, 1 are observed. Extrapolation of the 2( term energies for both isotopomers to a common origin places the electronic origin of the (X) over tilde (1)A'->(A) over tilde (1)A '' system near 10 500 cm(-1), in good agreement with theoretical predictions. Rotational analysis of the 16 observed bands for CHI and 13 observed bands for :CDI yields rotational constants for the upper and lower states that are also in good agreement with theory. To investigate the controversial issue of the ground state multiplicity of :CHI, we measured single vibronic level emission spectra from many (A) over tilde (1)A '' levels. These spectra show conclusively that the ground state is a singlet, as for both isotoporners the (a) over tilde (3)A '' origin is observed, lying well above the origin of the (X) over tilde (1)A' state. At energies above the (a) over tildeA '' origin, the spin-orbit mixing is so severe that few vibrational assignments can be made. Analysis of the emission spectra provides a lower Iii-nit on the singlet-triplet gap of 4.1 kcal mol(-1), in excellent agreement with theoretical predictions.