The presence of defects in adhesive materials influences the mechanical behaviour of bonded composite structures. Indeed, the initial defects in the adhesive joint damage the mechanical properties of the interface and reduce the mechanical strength of the structure. An in-depth study of the influence of initial porosity in the bonded joints on the mechanical properties of the rigid adhesive material has been experimentally measured. Different joint geometries are investigated by varying the thickness and the bonding area. A modified Arcan device is used to load the bonds in tension (initiation in Mode I). An imperfect interface model capable of considering the presence of initial diffuse cracks according to the Kachanov-Sevostianov material definition is then used to predict the mechanical properties of these adhesive joints. In this model, the presence of defects in the adhesive layer is described by an initial damage parameter: the porosity rate. Prior to testing, the porosity rates of the bonded joints are measured using µCT scans. Finally, the mechanical properties obtained experimentally are compared with those predicted by the imperfect interface model.