Reactions of laser-ablated Cr atoms with O-2 gave a very strong, sharp 965.4 cm(-1) band and weak, sharp 1869.7, 984.3, 914.4, 846.3, 716.2, and 643.1 cm(-1) bands. The 1869.7, 965.4, and 914.4 cm(-1) bands track together on annealing, show Cr-52, Cr-53, Cr-54 isotopic splittings appropriate for a single Cr atom and triplets with statistical O-16,18(2) for two equivalent O atoms, and are assigned to the upsilon(1) + upsilon(3), upsilon(3) and upsilon(1) modes of the bent (128 degrees+/-4 degrees) chromium dioxide OCrO molecule. The 984.3 cm(-1) band shows chromium isotopic splittings for two Cr atoms and O-16,18(2) components for two O atoms, and is attributed to the bent CrOCrO molecule. The weak 846.3 cm(-1) band exhibits proper oxygen isotopic behavior for CrO and is redshifted 39 cm(-1) from the gas-phase value, the maximum shift observed for a first row transition metal monoxide. The sharp 716.2 and 643.1 cm(-1) bands crack together; the former reveals Cr isotopic splittings for two Cr atoms and the latter O-16,18(2) splittings for two sets of dioxygen subunits; the branched-puckered-ring dimer O(Cr2O2)O is identified. Annealing produces new bands due to CrOO, CrO3, Cr(OO)(2) and the ring dimers (Cr2O2) and (Cr2O2)O, which are identified from isotopic shifts and splitting patterns. (C) 1997 American Institute of Physics.