The infrared spectra of monomeric and dimeric aluminum chloride trapped in solid argon have been observed. For the monomer AlCl3 four infrared-active fundamentals including the symmetric stretch are observed, implying a pyramidal equilibrium structure. Other experimental data for AlCl3 are consistent with such a structure. A normal coordinate analysis suggests a Cl-Al-Cl angle near 112°. The structures of the aluminum halides are compared with observed trends in alkaline-earth dihalides.