Matrix studies featuring the FTIR instrumental advantages of resolution and sensitivity for a finite amount of time are described. Ammonia clusters have a complicated spectrum in the N–H stretching region. One antisymmetric and two symmetric stretching modes in the dimer verify an asymmetric cyclic dimer structure.1 Chlorine isotopic splittings have been resolved for the four H/D substituted (HC1)3 species, which confirm the cluster stoichiometry and triangular structure.2 The ammonia/hydrogen cyanide system reveals 1:1 and 1:2 complexes, which are identified from their FTIR spectra and characterized by comparison to similar complexes.3 The ozone/hydrogen fluoride system gives the O3–HF complex, and mixed oxygen isotopic spectra show inequivalent terminal oxygen atoms.4 The symmetrical PH3–O3 complex photolyzes to give an asymmetric HOPO2 species based on mixed isotopic spectra.5