It was found recently that boards of mats and prints mounted in them had a brown discoloration within the window opening and that this browning occurred through a whole stack. The mats had been kept in good quality cardboard boxes in a well climatized storage room. Two hypotheses for the cause of the discoloration were put forward, both based on oxidation of cellulose during changes of the relative humidity in the opening. The first hypothesis stated that degradation of cellulose occurs due to evaporation of water at a wet-dry interface. In the second hypothesis condensation of water due to changing temperature is the critical factor. The changes in relative humidity within a mat in a box were measured in a climate chamber, along with the temperature, using electronic data samplers. In the first experiment the temperature was kept constant, while the relative humidity was being lowered and raised alternately. These changes hardly had any effect on the relative humidity within the mat. In the second experiment the relative humidity was kept constant while the temperature was changed. Both the temperature and the relative humidity inside the mat reacted rapidly to the external changes. There was an increase in relative humidity at higher temperature and vice versa. Furthermore the difference between hard and soft sized board was examined. It was found that the relative humidity changed more and swifter in the opening of a soft sized board than of a hard sized board under the same conditions. The water absorption rate was determined using the Cobb test and by the absorption and desorption plots. The possibility of chemically determining differences between the discolored and the unexposed original part of the board was also investigated. Microchemical tests and FTIR were applied to find changes in the cellulose. The presence of radicals, formed in the degradation process, was tested using the oxidation-sensitive dyestuff N,N'-diphenylguanidine (DPG). In order to cause the discoloration artificially, the temperature in a climate chamber with a mat was changed in four-hour cycles between 60 and 80°C, at constant relative humidity (60%), for three weeks. The resulting discoloration in the mat was investigated chemically and with FTIR and proved to be caused by oxidation using the DPG test. Several theories about the reaction mechanism of the oxidation were examined. -- AATA