Polyamide 11 is used as a leakproof sheath inside flexible flowlines for petroleum products. Under some operating conditions, this polymer undergoes a degradation of its original physicochemical and mechanical properties, which may be assimilated with a phenomenon of aging. Material exchanges occur between polyamide 11 and the fluid transported. The components present in the fluid (water, hydrocarbons) may be absorbed, and the principal additive of the material (the plasticizer) is extracted. This study was carried out to determine the influence of the composition of the chemical environment of aging on the properties of polyamide 11. In the first phase, a new analysis method was developed for quantifying diffusing materials in polyamide 11. Effectively, several techniques can be used for determining such materials. however, interference problems may be encountered when the polymer is in contact with oil containing sulfur-bearing products. Likewise, none of these techniques is capable of simultaneously making a complete analysis of all the materials. The principle of the method developed consists in performing a thermodesorption of the different materials present in the polymer and in analyzing them on line by medium-resolution mass spectrometry (resolution = 3000). This resolution is also capable of determining the distribution, by chemical families, of the hydrocarbons absorbed. The method was checked with aged polyamide 11 samples containing either a single type of material or several materials. These materials were analyzed at the same time by thermodesorption coupled with mass spectrometry and by other techniques. The results obtained show that this new method is capable of quantitatively and simultaneous determining: (1) the residual plasticizer content, (2) the water and absorbed-hydrocarbon content, and (3) the distribution, by chemical families, of the absorbed hydrocarbons (alkanes, cycloalkanes, aromatics). In the second phase, since polyamide 11 has varying affinity for the different chemical families present in a petroleum environment, we tried to determine the influence of each type of compound on the mechanical properties (ultimate elongation) and physicochemical properties (rate of crystallinity, molecular weight, contents of diffusing materials). Aging was performed in the laboratory in model environments made up in varying proportions, i.e. water, a gas-oil cut composed mainly of aromatic hydrocarbons, a gas-oil cut composed mainly of aliphatic hydrocarbons, The composition of these different aging environments was chosen by means of an experimental plan applied to the mixtures, and a polynomial mathematical model has been postulated. The conditions of temperature, pressure and aging time were set at a constant level for all the tests, i.e. 140-degrees-C, 6 bar of nitrogen, 15 days. The model obtained was used to plot isoresponse curves and to predict the properties analyzed as a function of the composition of the environment. The influence of water on the degradation of the mechanical properties (decrease in ultimate elongation) and the physicochemical properties (reduction of the molecular weight and increase in the rate of crystallinity) was revealed. Likewise, it was shown that, no matter what the aging environment was at 140-degrees-C, the plasticizer is extracted and the aromatic environment influences the plastication of the material. By means of mass spectrometry, the chemical nature of the aromatic hydrocarbons absorbed preferentially, and having a pasticating role, was determined. These are two-cycle aromatics (alkylnaphthalenes, acenaphthenes, diphenyls, acenaphthylenes, fluorenes) and sulfur-containing aromatics (benzothiophenes and dibenzothiophenes). The different results led to the conclusion that the principal phenomenon involved in aging is a hydrolysis caused by the presence of absorbed water in the material, which leads to the cutting of the macromolecular chains and embrittlement of the polymer. Lastly, the model created with gas-oil cuts was applied to a case of aging of a crude oil, and this led to the satisfactory prediction of how the mechanical and physicochemical properties of the polymer evolve in such an environment.