Water vapour sorption processes on mixtures of soil and organic fertiliser Rosahumus (RH) were investigated. Mixtures were prepared with Orthic Luvisol soil formed from loamy sand and Haplic Luvisol soil formed from loess. Samples contained different doses of fertiliser in the range from 1 to 50 weight percent. Differences in water vapour sorption on Rosahumus (RH) as well as the soils were observed. The properties of soil mixtures and organic material depended on the properties of both components and on the dose of Rosahumus. However, fertiliser was a dominant factor in sorption processes of investigated mixtures. Adsorption isotherms obtained by gravimetric method belonged to the same type II, in accordance to the BET classification. Significantly higher influence on the water vapour sorption was associated with fertiliser dose and was greater for mixtures with soil formed from loess. The increase of water vapour sorption was visibly higher for mixtures below 4% of Rosahumus content. On the other hand, greater concentration of RH resulted in similar amount of water vapour sorption and for Rosahumus dose of 50% the sorption was practically the same. Similar results were found for the maximum hygroscopicity (Mh) of mixtures. Strong correlation was observed between the Mh and the amount of added organic fertiliser with determination coefficient (R²) from 0.997 to 0.998.