Tempering the martensitic T91 steel at 550 °C instead of 750 °C (recommended heat treatment) makes T91 more sensitive to liquid metal embrittlement. Especially, in liquid sodium, T91 tempered at 550 °C exhibited a brittle fracture while it was ductile in air. Liquid sodium initiated an intergranular crack which then propagated by transgranular mode in the bulk of the specimen. ToF-SIMS analyses were performed on dedicated Small Punch Test specimens in order to distinguish the contribution of the mechanical loading from the sodium environment. Controlled elastic or plastic deformations on pre immersed (503 °C during 169 h) and on as received materials were performed in liquid sodium at 450 °C. Sodium penetration at prior austenitic grains boundaries and at laths boundaries has been pointed out by TOF-SIMS. It is concluded that the sodium penetration requires plastic deformation.