Foxing on paper is a common phenomenon that damages the aesthetic value of books. It is difficult to remove foxing thoroughly and thus it is hard to restore the original appearance of the book. An understanding of the formation of foxing is beneficial to its prevention. The origin of foxing has not been clearly explained due to its complexity. This study investigated the state of the foxing contamination in books collected by the Shanghai History Museum and provides information on preservation and conservation of modem books and other early machine – made paper relics. The results showed that nearly 60% books have foxing. There is some correlation between foxing and paper types, the pH value of paper. Machine - made paper (especillay newsprint) , with a relatively low pH value has more severe foxing than handmade paper.