When a Ne:HCl or a Ne:DCl sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the solid deposit includes the fundamental absorption of HCl+ or DCl+, which appears about 1% below the corresponding gas-phase band center. Another absorption, intermediate between the fundamentals of HCl and of HCl+, is contributed by an HCl-stretching fundamental of (HCl)(2)(+). Among the important anion species present in the solid is C1HCl(-), infrared absorptions of which are identified. Charge delocalization is sufficiently reduced in solid neon, compared to the heavier rare gases, that ion production from HCl does not occur at or below 10.2 eV. The electric field of the ions trapped in solid neon inhibits the rotation of HCl and leads to the appearance of a prominent HCl Q-branch absorption. In the presence of traces of oxygen, the O-2...HCl+ complex is stabilized, as evidenced by the appearance of the OO- and HCl-stretching absorptions of that species. The two O atoms are equivalent or nearly equivalent in the complex. It is suggested that photoexcitation of O-2...HCl+ leads to charge exchange, producing O-2(+) and HCl, with a threshold near 370 nm.