The magneto-optic Kerr effect in-situ has been used to investigate the effects of exposing ultrathin Co/Cu(llO) films (6ML<30ML) to CO gas and the subsequent effects of depositing submonolayer quantities of Cu. After a given deposition of CO, the adsorption of CO gas (but, not O2, H2) is found to cause the magnetic easy axis to switch 90° from the [00l] to the [1-10] direction. The deposition of submonolayer Cu overlayers are observed to reverse the effect of the adsorbed CO thereby switching the easy axis back to [00l]. For thin CO films (dCo<15ML), Cu overlayers switch the magnetic easy axis back to the [00l] direction abruptly. In thicker CO films (>15ML) the easy axis is found to shift gradually from the [l-10] to the [00l] direction, and it allows us to controllably engineer the direction of the magnetic easy axis at a constant CO thickness.