A composition and method for treating fibers is based on a mixture of an organopolysiloxane having at least one amino-substituted hydrocarbon radical directly bonded to a silicon atom and a higher fatty carboxylic acid. The carboxylic acid reacts with the amino radicals to reduce yellowing and oxidation of the fiber treatment. The composition and method provide non-yellowing fibers and a treatment agent that does not gel during use, such as when exposed to carbon dioxide and/or used to treat carbon fibers.