OBJECTIVE The fracture toughness (KIC) and load relaxation of four dentin bonding resins were determined to characterize some of the mechanical properties of these materials after polymerization. METHODS A total of 40 single-edge notch bar specimens were fabricated, 10 each of four commercially available brands, and subjected to three-point bending until fracture, as described in ASTM Standard E399-83 (1991a). The critical stress intensification factor, KIC, was derived for each specimen and compared by analysis of variance and Scheffé's multiple comparisons test (p < 0.01). To study the load relaxation characteristics, five rectangular specimens (without notches) of each brand were subjected to three-point loading until a predetermined limiting load value was reached. The test load was allowed to relax for 4 min, after which the specimen was unloaded to the zero load condition, and the load was allowed to build up on its own accord for 3 min. Load relaxation values were measured from the chart, and the mean percent load drop was calculated. The load relaxation data were compared using analysis of variance and Scheffé's multiple comparisons test (p < 0.05). RESULTS The fracture toughness (KIC) values of the four adhesive resins studied in this investigation ranged from 0.37-0.94 MPa.m0.5 and were statistically different from each other (p < 0.001). The load relaxation values were found to be greatest within the first 0.5 min, with the total load relaxation of the four bonding agents ranging from 16%-30%. Two of the materials studied showed significantly different short-term load relaxation behavior than the other two resins (p < 0.05). CONCLUSIONS Bonding agents can be implicated as one of the factors that weaken the interface between the dentin and the composite restorative material. These materials are capable of a rapid short-term response, demonstrating significant load relaxation in the first 0.5 min after loading.