This work aimed to study the absorption and desorption behavior of some clay–sandy plasters reinforced with natural fibers that could be used for straw bale buildings. The plaster materials consisted of soil, sand and chopped straw. Straw is used as a reinforcement fiber for plaster. Three types of fibers were used, namely wheat straw, barley straw and sawdust. The desorption behavior of plaster materials was tested at different temperatures of 30°C, 50°C and 70°C. The results showed that the plaster reinforced with fibers dried slower compared to those that have no fibers. On the other hand, faster drying led to cracks in the plaster, which is not desirable in building surface coating. The moisture absorption rate increased with increasing fibers content and decreased with increased sand content. Moreover, the highest value for the moisture absorption rate was obtained for the plaster reinforced with sawdust, whereas the lowest value of moisture absorption rate was obtained for the plaster without reinforcement fibers.