A total of 15 bands of HCCCN and 10 bands of DCCCN, occurring in the region 1800–6500 cm-1, have been recorded at resolutions of between 0·025 and 0·050 cm-1. Sufficient fundamental, overtone, combination and hot bands have been analysed to enable accurate band origins to be determined for v1, v2, v3, v5, v6 and v7 of both isotopic species and to enable estimates of the band origins of v4 of both isotopic species to be made. The rotational structure of 37 bands has been analysed to yield accurate values of vi0, (B′-B″) and in some cases, (D′-D″).