A method is presented for determining the elastic constants E (Young's modulus) and v (Poisson's ratio) for a general adhesive in situ. The method was applied to the structural adhesive FM-300K at room temperature, giving the values E = 2504 MPa and v = 0.34. These constants were then compared with the corresponding constants obtained from an adhesive bulk specimen test, E = 2330 MPa and v = 0.39. The two sets of constants were studied with regard to their sensitivity to scatter in the test results. The study indicates that the in situ method is more stable than the bulk method. Finally, different combinations of elastic constants were used when analyzing scarf joints with different scarf angles using the finite element (FE) method. The scarf joint geometries analyzed were also manufactured and tested, giving large scatter in the elastic stiffness. A comparison of the FE results and the mean values of the test results indicates that the two sets of constants predict the elastic stiffness of the scarf joints with reasonable accuracy.