Abstract A normal coordinate calculation has been carried out for cyclopropenone- d 0 and - d 2 to try to understand the origin of their strong bands at 1483 and 1409 cm −1 respectively. The conclusions are as follows. (1) The five a 1 frequencies for each molecule (i.e. 10 in all) can be reproduced well with only five diagonal and one off-diagonal force constants. (2) The 1483 and 1409 cm −1 bands are the CC stretches. About 80% of their potential energy is in this coordinate. They are the lowest localized CC stretches known to the authors. (3) The force constant for the CC stretch is lower than usual, whereas that for the CC stretch is higher. (4) In disubstituted cyclopropenones, the intense bands near 1850 and 1650 cm −1 are strongly mixed CO and CC stretches. The 1650 cm −1 one is not a pure CC stretch at all; it is an in-phase mixture of approximately equal amounts of the above two coordinates. The 1850 cm −1 band is the out-of-phase mixture, with some added CC symmetric stretch. (5) The striking difference between the apparent “CC” frequencies in cyclopropenone- d 0 and - d 2 (<1500 cm −1) and those in disubstituted derivatives (∼1650 cm −1) is due almost entirely to a mass effect.