Using a high‐energy single‐pulse DF laser, a multiple‐pulse effect (MPE) occurs for aluminum alloys in air. It requires an incident fluence well above the plasma threshold fluenceeT. After a number of exposures, the thermally coupled fluence is almost three times the plasma‐enhanced value seen for single pulses aboveeT. The absorptance beloweTat 3.8 &mgr;m increases by a factor of 5. We now find that the MPE depends on the presence of O2. New experimental data were taken well aboveeTin air at low pressures and in gas mixtures at 1 atm with varied O2content. Plasmas were present but the MPE was not for low O2. The normal single‐pulse enhancement was absent without O2.