Onion is one of the oldest and most consumed crops in the world. Its quality deteriorates gradually during prolonged storage. In the present study, white onions of the variety "Sierra Blanca" were stored in a controlled atmosphere (CA) for 7 months. The color, total pungency, microbiological load and total carbohydrates were evaluated. Three treatments were applied: control treatment (CT), regular atmosphere at 2.5 °C without controlled relative humidity (RH); (T1)1% O + 1% CO at 2.5 °C and 60-75% RH and (T2)3% O + 5% CO at 2.5 °C and 60-75% RH. The statistical analysis showed a significant effect of the treatments and storage time in most of the variables analyzed. The CA managed to preserve the quality of the onion during the storage period while the control bulbs exhibited greater deterioration. These increased 2.9 times their level of pungency, had a higher microbial load and showed a very noticeable color change at the end of storage with respect to freshly harvested onions and those stored in a CA. They had a more yellow-green coloration. Both CA treatments were effective in preserving onion quality, but a more positive effect for T1 than T2 was observed on most of the parameters evaluated.