In this paper, the effects of hydrogen charging and aging treatment on the cyclic plasticity of a S32304 lean duplex steel were investigated. The surface modifications were analyzed using atomic force microscopy and scanning electron microscopy. The slip morphologies of annealed, hydrogenated or thermally aged specimens were compared after cyclic loading at p= 0.07%. Significant changes were detected for austenite charged with hydrogen and for ferrite thermally aged. The results indicated a planar slip mode with a secondary slip for hydrogenated austenite. The height and the spacing between the slip marks of austenite (SBH and SBS) decreased from [30-5] nm to [8-2] nm and from [3-0.7] m to [1-0.2] m respectively with hydrogen addition. The morphology and the mean value of SBH and SBS of ferrite slip lines remained the same after 5 h of hydrogen charging. However, the increase of hydrogen charging duration created micro-cracks inside the slip bands of ferrite and decreased the value of their SBH and SBS. The thermally aged steel developed straight slip bands inside the ferrite after cyclic loading while they were curvilinear in the annealed material. The effect of the -phase spinodal decomposition was pointed out even for a short aging duration.