Custom 465 is an advanced precipitation hardened martensitic stainless steel exhibiting a combination of high strength, high fracture toughness and good corrosion resistance. This steel is recommended for use in hydrogen atmospheres, yet only little research has been published on hydrogen behavior in this alloy. Here, the diffusivity, solubility and average detrapping energy for hydrogen were compared in various thermal conditions (solution annealed, H900 and H1000), employing electrochemical permeation and thermal programmed desorption measurements. It is suggested that reversible (low energy) traps in the H900 and H1000 conditions, associated with (semi)coherent η-Ni 3 Ti precipitates, are responsible for the high hydrogen solubility and low diffusivity. At the peak of coherency of the precipitates in the H900 condition, higher solubility and lower diffusivity and detrapping energy were measured. The value of the diffusion coefficient is found to change during different stages of charging and discharging, depending on the level of occupancy of the reversible traps. Carpenter Technology's Custom 465 (UNS S46500) is an advanced precipitation hardened (PH) martensitic stainless steel, characterized by a combination of high strength and fracture toughness with good corrosion resistance, compared to other PH stainless steels. 1-3 Although this steel is recommended for engineering applications in corrosive environments that contain hydrogen, only little work has been published on hydrogen interaction with this steel, to the best of our knowledge. [4][5][6] Hydrogen diffusivity is affected by the microstructure that results from different thermal treatments. 4 The microstructure of Custom 465 steel 7 in the solution annealed (SA) condition consists of Fe-Ni martensite, which is characterized by blocks of parallel lamellae organized in packets along the prior austenite grains. During aging, η-Ni 3 Ti nano-precipitates are formed inside the lamella, and reverted austenite at grain boundaries and interlamella boundaries. For engineering applications, both H900 and H1000 age conditions are mostly recommended. 1 In the H900 (482 • C/ 4 hours) age condition, 7 the (semi)coherent rod-shape nanoprecipitates of η-Ni 3 Ti, about 3 nm in diameter and 8 nm long, are formed, and a maximal ultimate tensile strength (UTS) of about 260 ksi (1793 MPa) is obtained. In the H1000 (540 • C/ 4 hours) age condition, precipitates grow to about 9 nm in diameter and 27 nm in length. Due to the lower coherency of the precipitates and their lower density in the H1000 condition, the UTS decreases to 225 ksi (1551 MPa). At the same time, the ductility, fracture toughness, general corrosion and stress corrosion cracking (SCC) resistance all improve. 1 In both the H900 and H1000 age conditions, the reverted austenite concentration is less than 5 vol.%, and it is dispersed as discrete particles. 7 The fundamental correlation between the microstructure change during aging and hydrogen behavior in PH martensitic stainless steels has been studied to li...