The microstructural characterization of two titanium alloys, Ti5Al2.5Sn and Ti6Al4V, was performed. These alloys were selected as representatives of the alpha and alpha + beta titanium families, to investigate their possible application as structural materials in fusion reactor technology. Scanning and transmission electron microscopy have been used to obtain the relevant features of the microstructure in the as-received state and after mechanical deformation in tensile and fatigue experiments. Deformation modes and defects resulting from the deformation process were analyzed. Deformation occurred in the alpha phase of both alloys mainly through slip of the basal dislocations in basal, prismatic and pyramidal planes, together with slip in the (c + a) direction, A layered structure of dislocation loops developed in the alpha phase after cyclic fatigue tests. Deformation induced a phase transformation, namely a martensitic phase transition, that occurred solely in the 0 phase present in the Ti6Al4V alloy. (C) 2002 Elsevier Science B.V. All rights reserved.