Nowadays, there is an increasing interest in using fibre reinforced polymers (FRP) for strengthening masonry elements. It has been observed that these materials, when used for externally bonded reinforcement (EBR), improve the performance of masonry components. However, issues such as durability and longterm performance of strengthened elements are still open. The bond between composite material and masonry substrate is a critical mechanism in EBR strengthening techniques, and therefore its durability and long-term performance should be deeply investigated and characterized. In the present study, the influence of hygrothermal conditions (combined effect of moisture and temperature) on the bond performance is investigated by performing single-lap shear bond tests on two sets of GFRP-strengthened specimens. Different surface preparation techniques are used for each set of specimens to study their effects on the bond degradation. The effect of boundary conditions on FRP delamination is also investigated through visual inspection. The results showed surface grinding significantly improves the bond durability. Extreme FRP delamination was observed in specimens prepared with original brick’s surfaces, being mainly due to thermal incompatibility.