Based on the JKR and DMT models, dynamic contact stiffness of a rigid sphere against an adhesive semi-infinite solid is investigated by the consideration of dynamic contact deformation at the contact interface. The assumption of sufficiently small oscillating force yields a dynamic contact-pressure distribution of constant contact size, and then dynamic contact stiffness. It is found that except for the contact radius, two adhesive models predict the same expression of quasi-static contact stiffness and dynamic adhesive contact stiffness factor (DACSF). The influence of the oscillating frequency and specimen elasticity on the DACSF is discussed. (C) Koninklijke Brill NV, Leiden, 2011