Pulsed-laser ablated Ca, Sr, and Ba atoms have been reacted with O-2 in excess argon during condensation at 10 K. Infrared spectra and oxygen isotopic substitution show that the major products are the symmetrical OMO metal dioxide and the rhombic metal oxide dimer. The metal oxides and cyclic MO(2) peroxides are minor products. In the case of calcium, activation energy provided by pulsed-laser ablation makes the reaction to give CaO, CaO2, (CaO)2, and OCaO possible. Strontium and barium are more reactive and the dioxides and peroxides increase on annealing the argon matrix to 25 K, which shows that no activation energy is necessary for the Sr and Ba atom reactions with O-2. Photolysis decreased the peroxide in favor of the dioxide species. The alkaline earth metal peroxides are characterized by O-O stretching absorptions in the 700-cm(-1) region and are distinctly different from the alkali metal superoxides, which are characterized by O-O stretching absorptions in the 1100-cm(-1) region.