Water activity (aw) is a property of great relevance in the food area for both research and industry because it is directly related to food stability due to degrading processes of physical, chemical, biochemical, and microbiological nature. The most practical method for determining the aw of food is the direct measurement in the water activity meter. However, many research laboratories and industries do not have such equipment due to financial limitations. In these cases, using a moisture sorption isotherm (MSI) of the product of interest is an alternative for indirectly determining aw. In this sense, this chapter presents an alternative method: simple, low-cost, and fast for determining MSI.