Various types of interlocking mortarless (dry-stacked) block masonry system have been developed worldwide. However, the characteristics of dry joints under compressive load, and their effect on the overall behaviour of the interlocking mortarless system, are still not well understood. This paper presents an experimental investigation into the dry-joint contact behaviour of masonry and the behaviour of interlocking mortarless hollow blocks for grouted and ungrouted prisms under compression. Two experimental test set-ups are proposed to evaluate the contact behaviour of dry joints, considering the geometric imperfections in the contacting faces. The results show that the contact behaviour of a dry joint is highly affected by geometric imperfections in the block bed. Different patterns of deformation are distinguished in mortarless hollow (ungrouted) and grouted prisms. Dry joints predominantly affected the hollow prism deformation until the compressive load reaches 0·57 of the maximum load. However, this behaviour is not common in grouted prisms, because noticeable deformation commences after 0·38 of the maximum load. Furthermore, the variations of strength and deformation in grouted specimens are diminished compared with those in ungrouted specimens. © 2006 Thomas Telford Ltd.