The infrared spectra of trans- and cis-[bis (D-alaninato) copper (II)]_n and their isotopic complexes containing ⁶³Cu, ⁶⁵Cu and deuterium were measured in the region between 4000 and 200 cm^-1. By referring to the isotope shifts on deuteration and ⁶³Cu and ⁶⁵Cu substitution, the bands around 370 and 320 cm^-1 were assigned to the Cu-N and the Cu-O asymmetric stretching vibrations, respectively, for both the trans and cis complexes. The magnitudes of the copper isotope shifts are consistent with those calculated from the normal coordinate analysis by using a complete molecular conformation and an intermolecular force field. The trans-isomer showed three copper isotope sensitive bands, while the cis-isomer showed two.