The first singlet-singlet n*-n transition of thiocarbonyl chloride CSC12 gives rise to a system of bands whose origin occurs near 5340A. Vibrational structure associated with the 35C1,CS and 35CWClCS isotopes has been analyzed in detail. As predicted by m.0. theory, the pure electronic jump is forbidden as an electric dipole transition, eA24wA1, and the bands observed are mainly vibvunic &*A1and A2-231 combinations. The organization of the spectrum is similar to that of the corresponding band system of formaldehyde, the resemblance being accentuated by the fact that the 1A2 electronic state of thiocarbonyl chloride is non-planar with sinlilar geometry to the 1Az state of CH20.Five (out of six) excited state fundamental frequencies, one of which is strongly anharmonjc, are identified by the isotope effect and other means. Two possible excited state structures are obtained from Franck-Condon considerations.* In 37Cl2CS, a and b are probably reversed ; but this isotope contributes only about 6 % to the total intensity. * It is always possible to construct a reference-plane (yz) through or parallel to the three atoms C1, S , C1, so that any configuration of the molecule can be represented in terms of orthogonal internal displacement-coordinate y,z, in-plane and x out-of-plane based on an origin at, say, the centre of gravity. Whether the x-coordinates of the carbon atom at any instant are positive (above the y,z plane) or negative (below the y,z plane) depends on our arbitrary choice of the positive direction. Any function of the x-coordinate representing a physical observable, e.g., a vibrational potentialfunction, must therefore be independent of our arbitrary choice of positive direction, i.e., a totallysymmetric even function of x : reflection (T in the y,z plane is one of the symmetry-operations of the system. This is independent of whether the planar configuration (x = 0) happens to coincide with a potential minimum or maximum. If the former, the molecule is planar ; if the latter, pyramidal.Similarly, any non-symmetrical in-plane configuration can always be described in terms of orthogonal (y,z) symmetry displacement-coordinates S,, S, based on a configuration in which a two-fold rotation about the C-S bond exchanges indistinguishable chlorine nuclei, and the potential-function must again be an even function of S,, Sz, so that C ~( Z ) is also one of the symmetry-operations. In three-dimensional space, C ~ ( Z ) and ~( y z ) = ov generate the point-group CzV.