Supermartensitic stainless steels (SMSS) have been rather recently developed as a promising technological alternative and economic option, with good corrosion resistance and suitable mechanical properties, mainly in the applications related to the gas and oil industry. Welding of these materials plays a crucial role, influencing their resistance to stress corrosion cracking (SCC), toughness and weldability. Additionally, the post weld heat treatment (PWHT) adjusts the final properties of the weldments, based on microstructural evolution. The objective of the present work was to analyze the effect of different PWHT’s on mechanical properties of all weld metal SMSS deposits, obtained using two shielding gases in GMAW. Understanding of phase transformation during PWHT was the key of this work to explain the changes observed on these properties. Two all-weld-metal test coupons were prepared according to standard ANSI/AWS A5.22-95, using a 1.2 mm diameter tubular metal-cored wire. The shielding gases used were Ar-5 %He and Ar-18 %CO₂. The welds were made in the flat position and the nominal heat input used was of 1 kJ/mm. Solubilization, simple and double tempering PWHT’s were done at different times and temperatures. Chemical composition of “as welded” deposits were measured. Microstructural characterization was carried out by means of optical and scanning electron microscopy, and x-rays diffraction. Charpy-V notch impact property was determined as well as micro-hardness and tensile property for the different conditions. Results relating chemical composition and microstructure to Charpy-V toughness were discussed.