Step-scan Fourier-transform infrared (FTIR) absorption spectroscopy has been used to study large clusters of C2H2 and C2D2 formed in pulsed supersonic jets. The v(3) stretching bands in the infrared spectra of C2H2 and C2D2 have been measured near 3230 and 2400 cm(-1), respectively. The band positions show the formation of acetylene nanoclusters with the cubic solid structure. The deuterium isotopic shift is in good agreement with the predictions based on the harmonic oscillator model and confirms the assignment of the observed features to cubic C2H2 and C2D2. This is the first infrared observation of cubic solid C2D2, and the v(3) C-D stretching band was observed at 2399 cm(-1). Furthermore, deconvolutions of C2H2 spectra into Lorentzian components reveal a new feature at 3241 cm(-1) attributable to large amorphous clusters.