Water is the most critical limiting factor for decomposition processes in Mediterranean climate ecosystems. The aim of this study was to investigate the effects of increased soil moisture levels upon cellulose mass loss in an eastern Mediterranean ecosystem. We used a split plot experimental design to evaluate the effect of seasonality on the decomposition process by manipulating water availability in situ during different seasons. Nylon litterbags of mesh size 20 μm and 1 mm were filled with cellulose filter paper and placed on the plots during each season. The treatment consisted of a systematic uniform watering of the plots with 7 L/m2/week. Litterbags were randomly removed from each plot on a monthly basis. Cellulose mass loss showed differences between treatment and control plots and between seasons but was significant only in the spring. Cellulose mass loss was significantly affected by irrigation treatment, season, and their interaction, while moisture seemed to be affected only by season. There was no significant difference in cellulose mass loss due to different mesh sizes.