Thermal modification of wood causes decrease of pH value, causing problems while gluing such wood with different types of glues. To research the actual influence we have glued lamellas of 2 different stages of modification and one non-treated sample with 2 different glues, (UF and PF). A shear test (SIST-TS CEN/TS 13354:2004) has been used to test the mechanical properties, rheometer to test the rheological properties and dielectric analysis to measure glue curing. The results show that shear modules of a system (wood-glue) do not differ essentially when gluing with UF. However, gluing with PF shows bigger differences. Dielectric analysis results show higher curing velocity at the beginning of the test at heat treated samples. Shear strength is the best at heat treated wood and does not decrease when gluing with PF; in most samples it even increases. In our case the pH decreased while thermal treatment did not have any influence on curing capacity of PF glue. The glue used was too alkaline, so we could not influence the pH of the curing medium.