We report a new rotationally-resolved vibrational spectrum of the five-membered heterocycle thiophene (C 4 H 4 S). The spectrum was collected with a resolution of 0.00096cm −1 using the intense, spatially-confined infrared light produced by the Canadian Light Source synchrotron coupled to a Fourier transform spectrometer. The dense rotational structure of the ν 14 band of thiophene at 452.4145(6)cm −1 has been analyzed and the ground and excited state parameters have been determined based on the assignment of nearly 3000 c-type transitions corresponding to this ring-puckering mode.