The market of prebiotic foods, a category that includes the fibers, is growing because people are looking for healthy foods and beverages. The objective of this study was to evaluate the influence of the inclusion of two types of fibers (fructo-oligosaccharide – FOS and oat fiber) in two mixtures of orange powder beverage (normal and light) to evaluate its influence on the stability of these products when exposed at different relative humidities (RH). The products were evaluated for: moisture (dry basis - %db), water activity (Aw) and water vapour sorption isotherm at 25°C. Initially the beverages had moisture/water activity of 0,29%db/0,39Aw (normal) and 1.00%db/0,22Aw (light), the FOS 4.11%db/0,26Aw and oat fiber 8.65%db/0.52Aw. It was observed in the isotherms of beverages critical agglomeration in 53%RH (1.4%db in normal and 3.2%db in light) and dissolution only in the normal beverage after 69%RH (18.4%db). Total agglomeration was observed in FOS fiber after 43%RH (9.1%db) and dissolution in 53%RH (12%db). The oat fiber does not present agglomeration in all RH exposited. The normal beverage plus FOS fiber presented critical agglomeration in 53%RH (3.9%db) and started dissolution in 69%RH. In the light beverage + FOS fiber the critical agglomeration occurred in 43%RH (5.0%db) and dissolution in 58%RH which did not occur in light beverage without FOS. At beverage plus oat fiber, agglomeration was observed in the normal and light version in 63%RH (3.52%db and 6.39%db respectively) and dissolution only in the light in 84%RH. Thus, it was concluded that the incorporation of FOS in powder beverage increased the hygroscopicity of the two formulations and them this will require packaging with higher moisture barrier to preserve the product's characteristics during the shelf life. On the other hand, the addition of oat fiber in such products contributed to improving the stability of beverage powder mixtures.