The authors studied foxing on nearly 100 books to arrive at a closer definition of the phenomena and get some answers concerning the actual source of these discolorations and stains. After studying the concentration patterns of the stains in margins and in the text block, they propose to abandon the restriction of the term foxing to stains, and to extend its definition to the other more zonal forms of discoloration. Using photocopies of sheets, they conclude that the observed stains originated during the storage of the unfolded gatherings, i.e. before the binding of the book. Other types of stains were found to have developed after binding. Moreover, the use of more than one type of paper in the production of a book appears to cause sudden changes in the stain structures present. The physico-chemical process underlying the observed browning of humidified fibrous materials at the dry-wet interface, might play a crucial role in the discoloration of paper. The authors presume that this browning reaction takes place in paper at accumulations of moisture, caused by local condensation processes in the book. It can be expected that many factors influencing condensation and evaporation will play a role in this respect. Additional research is planned to verify the condensation theory experimentally.