Chlorocarbonyl(phenyl)ketene (2) undergoes a degenerate 1,3-shift of chlorine, as determined by C-13 NMR spectroscopy. The two carbonyl signals (183 and 157 ppm) coalesce at -30 degrees C, and from this as well as line-shape analysis, an activation barrier for the 1,3-Cl shift interconverting 2a and 2a' of Delta G double dagger = 41.8 +/- 4 kJ mol(-1) (10.0 +/- 1 kcal mol(-1)) is calculated. These data are in excellent agreement with calculated (G2(MP2,SVP) and B3-LYP/6311+G(3df,2p)//6-31G* + ZPVE) 1,3-Cl shift and rotational barriers. Analogous 1,3-halogen shifts in acyl isocyanates are predicted.