This article presents results of material characterization studies conducted on historical clay bricks and binder sampled from the brick masonry gallery of the southern quadrangle of the World Heritage Site of Vat Phou in Champasak, Province of Lao PDR. The primary objective of the sampling was to conduct micro-analytical laboratory studies on the binder in the brick masonry to establish the nature of the binding mortar used and to verify the presence of lime in it, if any. In addition, micro-analytical and physicomechanical tests conducted on the historic brick samples were used to establish compatibility with the newly manufactured replacement bricks for use in the restoration of the dilapidated brick gallery. X-ray diffraction technique was used to study the mineral composition in the system, and scanning electron microscopy provided images indicative of the material and binder characteristics. The study revealed that lime was not present in the binding mortar and it could possibly be a natural wood resin. The use of natural resins as a binder in masonry has been a popular theory in Southeast Asia, and these results provide first empirical proof of their use. This study also demonstrates how compatibility checks can be carried out between historic and new replacement bricks through analysis of their pore structure and physical properties. In addition, the test results established that lime was not a component of the ancient binder, which together with the evidence of the resinbased binder provides an important direction for restoration efforts of historical sites in the Mekong River Basin region covering countries such as Lao PDR, Cambodia and Vietnam. © 2020. All Rights Reserved.