Spectra of the weakly-bound N 2 O–C 2 H 2 and N 2 O–C 2 D 2 complexes in the region of the N 2 O ν 1 fundamental band (∼2224cm −1 ) are observed in a pulsed supersonic slit jet expansion probed with a tunable diode laser. Two bands are analyzed for each complex: the fundamental (N–N stretch), and a combination involving the intermolecular torsional (out-of-plane bend) vibration. The resulting torsional frequencies are 44.37 and 40.01cm −1 for the C 2 H 2 and C 2 D 2 complexes, respectively. This represents the first observation of the N 2 O–C 2 D 2 isotopomer, and the first direct determination of an intermolecular frequency for nitrous oxide–acetylene.