Previous work by the authors has shown that it is possible to measure transverse displacements of both faces of a cracked lap shear (CLS) specimen accurately. In this work, this approach is extended so as to determine experimentally the Mode I behavior of the delamination. By considering the difference in the displacements of the two faces, a surface opening displacement (SOD) can be calculated. This SOD is related to the crack opening displacement (COD). It is shown, using the finite element method (FEM) analysis, that the SOD is not identical to the COD, but is uniquely related to it, and is very sensitive to changes in the COD. Experimentally derived SOD profiles at different positions along the width of the crack front are compared with scanning electron microscope examinations of the sectioned specimen, and good agreement is observed. It is shown that there is delayed opening of the crack as the specimen is loaded. The absolute values of the experimental and FEM analysis SOD profiles are in reasonable agreement, and it is shown that GI values can be calculated from the experimental results. The method is cumbersome and the results are difficult to interpret. However, once the low load profiles are established, the method can be used to monitor subcritical crack growth easily. In all three material systems tested, considerable subcritical crack growth (up to 4 mm) was measured, and the associated G values are 15 to 40% lower than Gc.