The interaction between masonry units and mortar is a crucial factor for the quality of a wall. The most important factor is the adhesion between bricks and mortar in order to construct a masonry wall with adequate strength, good impermeability, and durability. In this work mortars were produced with various cement/lime/aggregates ratio. The adhesion properties of the mortars with clay bricks were tested with a simplified tensile/tear testing measurement. In order to investigate the adhesion properties in relation with microstructure the mortars were characterized with X-ray diffraction and were further investigated with scanning electron microscopy and stereoscopy. It was found that adhesion is favored by the formation of a Si-Al matrix with a low Ca content in the brick/mortar interface and the formation of fine Ca-Al-Si phases which can penetrate into the brick.