The far infrared spectrum of the four-membered heterocycle 3-oxetanone (c-C 3 H 4 O 2 ) has been recorded in the region 100–200cm −1 with a resolution of 0.000959cm −1 using synchrotron radiation from the Canadian Light Source. The ring puckering mode (ν 21 ) and first three hotbands were observed and 9165 separate rovibrational transitions were assigned and analyzed. The band centers of the fundamental band (139.549148(5) cm −1 ) and first three hotbands (v=1→v=2: 141.093457(14) cm −1 , v=2→v=3: 142.644034(19) cm −1 and v=3→v=4: 143.989670(26) cm −1 ) were accurately determined for the first time along with the rotational and centrifugal distortion constants that characterize the four excited vibrational states. Based on our spectra, the potential governing this vibrational mode of 3-oxetanone has the form of V=ax 4 +bx 2 (where a=2.04388×10 5 cm −1 /Å 4 and b=4.26392×10 4 cm −1 /Å 2 ) along the ring puckering coordinate x.