Compressed Earth Blocks (CEBs) have the ability to absorb moisture from the air, thus acting as a passive humidity regulator for the indoor environment. This capability is critical in humid environments, especially for people with respiratory problems, e.g., asthma. This study investigates the effect of the properties of the primary raw material (i.e., soil) used for producing CEBs in Cyprus on their sorption/desorption behavior. In this framework, an experimental investigation of various soil properties, e.g., particle-size distribution and mineralogical composition, was conducted. Furthermore, porosity and hygroscopic sorption/desorption isotherm experiments using the climate chamber method were carried out. The results of the sorption/desorption experiments confirmed that there is a hysteresis between the sorption and desorption curves, and a great variation between the response of the different CEBs investigated. This was attributed to the differences observed in the properties of the raw materials. Specifically, the CEBs produced with soils containing moisture-absorbing minerals absorbed a higher amount of moisture. Furthermore, some CEBs showed a greater hysteresis between their sorption and desorption curves due to the presence of clays in the corresponding soil that create strong binding forces with absorbed moisture. Additionally, the addition of sand in some soils led to a decrease in the amount of moisture absorbed, especially at 70% and 95% RH. This was attributed to the lower amount of moisture-absorbing minerals in the soil-sand mixture, compared to the raw soil, and the reduced porosity of the resulting CEBs. Thus, the raw materials used to produce CEBs appear to play a significant role in the hygroscopic sorption/desorption behavior of the end-products. This is highly interesting and may be further exploited to design customized CEBs for specific environmental conditions.