Yellow sweat stains occur when salt and aluminum from antiperspirants are combined, states About.com. Aluminum is the primary reason for the formation of yellow stains on clothing.
According to The Manual, antiperspirants in particular are the main cause of yellow stains, because the active ingredient in many antiperspirants is aluminum. Antiperspirants work by clogging sweat ducts to minimize odor and bacteria formation, while deodorant is responsible for destroying bacteria entirely. The aluminum, as opposed to salt from sweat, spawns a chemical reaction on clothing.
According to Natural News, some aluminum compounds found in underarm products are aluminum chlorohydrate, aluminum chloride and aluminum zirconium. However, The Manual does note that aluminum chlorohydrate is deemed the least harmful agent to clothing.
About.com further explains that yellow stains are most prominent on white shirts, but can also show on darker clothing as well. The best way to get rid of these stains is through careful washing, or dry-cleaning clothes that are not washable at home. One prevention technique is to allow underarm products to dry before putting on an article of clothing. According to the Art of Manliness, using bleach actually enhances the color of yellow stains on shirts. Type F recommends using aluminum-free antiperspirants to avoid yellow stains altogether.