The interaction between Cu atoms and ammonia molecules cocondensed in argon matrices was examined by ESR. The observed spectra revealed the formation of CU(NH3)(1-3) complexes. The g tensors and the hyperfine coupling tensors for the Cu and N nuclei of the complexes were determined. All three complexes showed large, essentially isotropic Cu hyperfine coupling interaction corresponding to 50-60% of that observed for isolated Cu atoms. No hyperfine structures due to protons were discerned. It is thus concluded that these complexes are formed merely by the dative interaction of the NH3 lone pair electrons with vacant Cu 4s/4p valence orbitals. The unpaired electron resides in a Cu 4s/4p-hybridized orbital projected away from the ligand(s). The ESR spectrum of the monoligand complex Cu(NH3) showed reversible photoconversion to the second spectrum in accord with the photoisomerization process Cu(NH3) <-> HCuNH2 reported in an earlier IR study. The second spectrum, though partially obscured by other signals, revealed a pattern consistent with the ionic character H-Cu2+(NH2)(-) where the unpaired electron resides in the Cu 3d(z)(2) orbital.