The objective of this study was to investigate the effect of different polymeric carriers in solid dispersions with an active pharmaceutical ingredient (API) on their water vapour sorption equilibria and the influence of the API–polymer interactions on the dissolution rate of the API. X-ray diffraction, scanning electron microscopy (SEM), moisture sorption analysis, infrared (IR) spectroscopy and dissolution tests were performed on various API–polymer systems (Valsartan as API with Soluplus, PVP and Eudragit polymers) after production of amorphous solid dispersions by spray drying. The interactions between the API and polymer molecules caused the water sorption isotherms of solid dispersions to deviate from those of ideal mixtures. The moisture sorption isotherms were lower in comparison with the isotherms of physical mixtures in all combinations with Soluplus and PVP. In contrast, the moisture sorption isotherms of solid dispersions containing Eudragit were significantly higher than the corresponding physical mixtures. The nature of the API–polymer interaction was explained by shifts in the characteristic bands of the IR spectra of the solid dispersions compared to the pure components. A correlation between the dissolution rate and the water sorption properties of the API–polymer systems has been established.