In this paper, the effect of several brick and mortar mechanical and microstructural parameters on the maximum debonding force of the same glass fiber reinforced polymer (GFRP), applied on different bricks and on corresponding masonry panels, was investigated. GFRP sheets were bonded by epoxy resin to four different types of solid fired-clay brick and four types of masonry panels, manufactured using the same bricks and a natural hydraulic lime mortar. The reinforced specimens were subjected to bond tests to evaluate the maximum debonding force. Bricks and mortar were characterized in terms of compressive strength (in the case of bricks, along two different orthogonal directions), surface roughness and pore size distribution. Based on the results of the study, alongside brick mechanical properties in different directions, also brick microstructural parameters seem to play a very important role, which should be taken into account for fully explaining the experimental results.