Determination of the fracture toughness properties and thermal aging behavior of dissimilar metal weld (DMW) joints is of utmost importance for successful structural integrity and lifetime analyses. This paper presents results from fracture resistance (J-R), fracture toughness (T0) and Charpy-V impact toughness tests as well as fractography performed for an industrially manufactured narrow-gap DMW mock-up (SA508-Alloy 52-AISI 316L). Tests were performed on post-weld heat treated, 5000 h aged and 10,000 h aged material. The results show that this DMW is tough at the SA 508-Alloy 52 interface, which typically is the weakest zone of a DMW. The DMW joint maintains its high fracture resistance also after thermal aging. Crack propagates for a large part in the carbon-depleted zone (CDZ) of SA 508 but deflects occasionally to the Alloy 52 side due to small weld defects in µm scale. Ductile-to-brittle transition temperature determined from Charpy-V impact toughness tests increases due to thermal aging, but only to a minor extent. No significant change is observed for the T0 transition temperature due to aging.