Cocondensation of Al atoms and HCl in argon matrices at similar to 4 K led to detection by ESR of HAlCl and AlCl2. The ESR spectra observed from the Al/HCl/Argon system are shown; assignments and analyses of the HAlCl and AlCl2 spectra are presented. Both species have a bent form, and the unpaired electron is localized in a nonbonding, sp-hybridized orbital of Al pointing away from the ligand atoms. Ab initio SCF calculations (Gaussian 92) were performed on AlH2, HAlCl, and AlCl2. The structural features and spin density distributions deduced from the observed hyperfine coupling tensors are in good accord with those given by the SCF calculations. The reaction process between HCl and Al was examined by the MNDO molecular orbital method. The MNDO study indicated that Al and HCl would undergo spontaneously either the insertion reaction, HCl + Al --> HAlCl, or the displacement reaction, HCl + Al --> H + AlCl, depending on the direction of approach.