Brown spots or stains on paper, books, or prints caused by a combination of fungal growth and metal oxidation in the paper’s pulp. Foxing is most common in books and prints from the 18th and 19th centuries and is generally considered a defect that reduces value.

Light foxing on text pages may be acceptable; foxing on plates, frontispieces, or dust jackets is a significant deduction. Conservation can sometimes reduce foxing, but never fully reverse it.