The objective of this work is the evaluation of hydrogen effects on the martensitic transformation and strain hardening in Duplex Stainless Steels (DSS) SAF 2205 (UNS S32205/S31803). DSS are two-phase alloys (austenite and ferrite), which are used for applications requiring high mechanical strength, in corrosive environments. Therefore, it is necessary a better understanding of the phenomena involved on the hydrogen embrittlement. For this, in situ measurements of X-ray diffraction were made during tensile test in H2 cathodically charging DSS 2205. The hydrogen charging reduces the stress relaxation, reducing the ductility and suppressing the hydrogen-induced austenitic to martensitic transformation. In addition, it also reduces the strain hardening (dislocation multiplication) in austenite. The strain hardening seems to have a higher influence than martensitic transformation on fracture process, even in absence of hydrogen.