For a flexible linear adhesive, the mode mixety of a single-layer adhesive joint is directly related to the deformation of the adhesive layer at the crack tip. The governing equations for linear elastic singe-layer adhesive joints show that the mode mixety depends on the exteranl loads, the properties of the adherends and often on the flexibility of the adhesive layer. This implies some fundemantal problems that has to be adressed before an experimental method can be established. Requirements for the design of a specimen configuration for a mixed mode testing of adhesive layers are given. A new specimen configuration is proposed and some preliminary experimental results are given.