The early stages of aggressive degradation at 85% of humidity for 24 h of single crystals of YBa2Cu3O7 − δ textured by Bridgman technique have been analyzed by means of atomic force microscopy, X-ray photoelectron spectroscopy, nanoindentation and focused ion beam. The results show that the initial formation of barium hydroxide and the non-superconducting green phase, Y2BaCuO5, induces the surface degradation and the loss of the mechanical properties as a consequence of the mechanical transformation after the degradation process due to an embrittlement of the superficial layer.