A thermodynamic approach to adhesion of polymer blends to solid is proposed. The different cases of compatible and incompatible blends are considered. The difference has been achieved between adhesion of polymer blends obtained by mixing two polymers in the region of two-phase melt and polymer alloys that initially by mixing are in the region of one-phase state and then, by changing temperature, transition into two-phase region. The mechanism of adhesion joint formation is different for polymer blends and polymer alloys, possibly related to phase transformation by cooling the melt in presence of a solid. The effects depend on the nature of the critical solution temperature. The conditions of the complete wetting the surface by one mixture component (or phase) and incomplete wetting by both components are established. General conclusions have been drawn as to the distinction between adhesion of polymer blends and alloys and adhesion of individual polymers to the surface. For polymer blends and alloys with complete wetting of the surface by one component, the interphase is formed by two layers: a first layer in direct contact with the solid surface (wetting layer) and a second layer between wetting layer and blend matrix. If adhesion between wetting layer and a matrix is lower than adhesion between wetting layer and solid, the failure of adhesion joints will take place in the zone of the contact between two blend components. After the phase inversion, there occurs a transition of the failure zone to the interface with solid. The transition from adhesion failure between the blend components to adhesion between the blend matrix and solid is a specific feature of adhesion of two-phase blends or alloys.