We have evaluated the resistance of two samples of TRIP 800 steel prepared under laboratory conditions at the Faculty of Metallurgy and Materials Engineering (FMME) VS. B (Technical University of Ostrava, Czech Republic) in a sour environment containing H2S. The first steel investigated had a C-Mn-Si composition, and the second steel had a C-Mn-Si-Al composition. Both TRIP steels were characterized using the yield strength in the range 420 to 450 MPa and tensile strength in the range 880 to 900 MPa. The TRIP steel samples were in the form of sheets with a thickness of 1.5 mm. The residual austenite content was 11% and 13%, respectively, in the two steels studied. The resistance to hydrogen embrittlement was evaluated in a sour environment that contained hydrogen sulphide using hydrogen-induced cracking (HIC) and sulphide stress cracking (SSC) tests performed in accordance with NACE standards. Both TRIP 800 steels showed a high resistance to hydrogen embrittlement, and no SSC cracks were observed. Some cracking arising from HIC was observed in both steels. The measured parameters showed some variation; in some cases they were lower than recommended limits, but in other cases the measured parameters were higher (e. g., the crack length ratio was up to 70%). The cracks initiated preferentially at non-metallic inclusions, either at elongated manganese sulphide particles, or at oxide stringers that were rich in Al.