The influence of voids on the quasi-static Mode I fracture performance of thick adhesive joints was investigated in this article. Double Cantilever Beam (DCB) joints were manufactured with glass fiber reinforced epoxy adherends and SikaPower®-830 epoxy adhesive in the cm thickness range. The highviscosity adhesive leads to the inevitable presence of voids, resulting in unstable propagation and promoting crack path deviations. Voids could also decrease the apparent fracture strain energy release rate (SERR) since the SERR calculation assumes that the fracture surface contains no voids, while the actual surface area is reduced because of the voids. In conclusion, grooved DCB joints with low void content tested at low displacement rates showed stable crack propagation without significant crack path deviations, allowing the estimation of reliable materials fracture data.