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Data on the phase concentration equilibrium are necessary to find out the direction of the process (drying or wetting of the material), calculate the kinetics of the drying process, and select storage conditions for the dried material. The paper presents the experimental isotherms of desorption of moisture from sunflower seeds of variety MAS 95.OL and from their components: kernels and shells—at temperatures of 24, 36, and 48°C. The studies were carried out by the static method using saturated salt solutions to create certain values of relative humidity in desiccators. It has been shown that seed shells have twice as much absorption capacity as kernels, due to differences in their morphological composition and structure. For the convenience of engineering calculations, the graphical equilibrium dependences were approximated by equations. By processing the obtained experimental data, a relationship is obtained between the equilibrium moisture contents of the nuclei and shells, which is linear in nature with a kink in the middle part of the equilibrium moisture contents. The boundary condition of mass transfer of the fourth kind (the condition of mass transfer conjugation) is given, which should be formulated when formulating the mathematical problem of drying a single seed that considers the seed as a two-layer body. [ABSTRACT FROM AUTHOR]