The 0–0 band of the [Formula: see text] system of propynal near 4145 Å has been photographed under high resolution and a rotational analysis carried out. The principal molecular constants for the ã3A″ state are (in cm−1):[Formula: see text]Altogether, 1237 lines have been assigned to transitions with ΔKa = 0. The dominant transition moment involves mixing of the ã3A″ state with higher 1A′ states.A simple magnetic rotation spectrum has been obtained and the assignments of the lines discussed.