This paper describes the assessment regarding the weldability of a 40 mm thick, supermartensitic stainless steel conforming to prEN 10088-2: X1CrNiMoCu1 2-5–2, which still profits from a lot of interest within the oil and gas industry especially for flow lines and process equipment of mild sour gas [1]. The emphasis in this study was placed on the low-temperature fracture behaviour of weldments made with up-to-date matching consumables applicable for pressure vessels, heat exchangers, slug catchers, etc. The weldability was evaluated by studying the influence on material properties of various thermal weld cycles, eventually followed by a post weld heat treatment (PWHT), typical for large pressure vessels where heat treatments very often are imposed by current codes. The investigation has shown, because of its low carbon, nitrogen and sulphur content, that the 1,5 Mo steel grade exhibits an adequate weldability and a superior response to PWHT. A close control of chemical composition and manufacturing route though is necessary to produce an economically attractive, weldable 13 Cr stainless steel having high strength in combination with satisfactory toughness and corrosion resistance. Submerged arc and gas metal arc welds, both adapted for longitudinal and circumferential welding, have been realised with consumables (over)matching in composition with the base metal. Also shielded metal arc welding, still the most widely used process for repairing, has been investigated. Several trials have been executed on all weldments to optimise the heat treatment. All weldments were assessed by performing standard mechanical testing, metallographic examinations, impact and CTOD fracture toughness testing and wide plate tests. Correlations between small and large scale test behaviour have permitted to advise the most appropriate post weld heat treatment for such thick supermartensitic stainless steel weldments and will allow formulating a first proposal for impact and/or CTOD fracture toughness requirements.