A stress singularity model is used to predict joint failures in single-lap joints with varying bondline thickness. The theoretical results are compared with experimental results for verification purposes. Essentially the experimental analysis is split into two parts. The first part determines the Young's modulus and Poisson's ratio of the adhesive selected for this verification study and the second part measures the failure loads versus bondline thickness of the single-lap joint specimens. Based on the experimental data on the adhesive properties, predictions are made of the joint strength (assuming failure is in the adhesive) for varying bondline thickness. It appears that the relation between the joint strength and bondline thickness can be described with a two-parameter Weibull function. The critical stress intensity factor, or bondline toughness, is derived from the test data using an approximation formula for the change in external loading with bondline thickness. The resulting critical stress intensity factor combined with the calculated stress intensity factors gives a good prediction of the joint strength over a practical range of bondline thickness.