The ageing of polyamide 11 (PA11) in water has been studied in the 53-210 degrees C temperature range, in media of pH 0, 2, 4 and 7, for sample thickness ranging from 0.5 to 2 mm. Ageing can be described as a succession of two stages. The first one corresponds to the physical absorption of water until equilibrium. During this stage, chemical degradation is insignificant. The second stage corresponds to the polymer hydrolysis where rate increases with temperature and solution acidity. However, the rate decreases with thickness, thereby indicating that degradation is diffusion controlled. A comparison of gravimetric (weight gain due to the reaction of water with polymer) and steric exclusion chromatography data indicates that most of the hydrolysis events occur at chain ends, and causes the accumulation of monomer and oligomers in the matrix. The presence of these groups can be detected by differential scanning calorimetry. Viscoelastic spectra reveal only a small plasticization effect from water absorption, implying that the monomer has a very poor plasticizing effect on PA11. Also, degradation is probably homogeneous at the scale of amorphous domains (no splitting of the transitions). Finally, cracks appear only in the superficial layers as a result of the large scale heterogeneity which results from the diffusion control of degradation kinetics. Embrittlement in these layers is presumably linked to the degradation of the entanglement network in the amorphous phase.