Yellow deodorant stains are caused by a reaction between the salts in perspiration and the aluminum compounds found in the majority of anti-perspirants. This type of stain is accumulative, meaning that it develops and intensifies over time. The longer such a stain is left untreated, the more noticeable it becomes.
To prevent yellow deodorant stains, use aluminum-free deodorant products. If such products cannot be obtained, or are not to your liking, the use of an undershirt or underarm sweat pads can prevent the problem.
Prevent staining by allowing deodorant to dry before putting on a shirt. This keeps the aluminum-based compounds from soaking into the fabric. The deeper the aluminum penetrates the fabric, the more difficult it will be to remove it through normal washing.
A solution of ammonia and water helps neutralize a fresh deodorant stain before it begins to accumulate and develop a yellowish appearance. Once a deodorant stain has set, it may require a pre-wash soaking to remove it. A solution of white vinegar, baking soda and water is ideal for this. Heat, such as that from a clothes dryer, also sets and exacerbates deodorant stains. Air- or sun-drying clothes susceptible to such stains is another preventative measure.