We investigated the behaviour in tension and at room temperature of TiNi shape memory alloys in martensitic state. Our first experimental results show that two deformation mechanisms are involved in the behaviour of such materials: martensite platelets reorientation and irreversible plasticity. Moreover when irreversible plasticity appears, we observe an increase in the martensite to austenite transformation temperatures. The present study is especially focused on the deformation mechanisms and intends to identify the nature of the irreversible phenomena. It is shown that the reorientation process and the appearance of interface dislocations are in perfect agreement with a kinematic hardening. Finally a two loading surface model is proposed which allows not only to account for the overall behaviour but also to predict the transformation temperatures under an internal and/or external stress field.