Six bands have been observed in the magnetic rotation spectrum of acrolein at 4040, 4069, 4122, 4178, 4220, and 4322 Å. The first five bands are assigned to the 3A″–1A′ (π*–n) transition of trans acrolein with the 0–0 band at 4122 Å. The sixth band at 4322 Å is assigned as the 0–0 band of the 3A″–1A′ (π*–n) transition of the cis (or gauche) isomer. The 3A″ state of the cis (or gauche) isomer lies below the corresponding state of the trans isomer by a similar interval to that found earlier for the 1A″ (π*–n) states. The reverse situation exists for the ground states.The absorption spectrum has also been reinvestigated using up to 30 m atm of gas in a cell heated to 180 °C. New assignments are given for some of the bands.