The sorption and the desorption isotherms of water are described for several crystalline polyamides (nylons 2, 3, 6, 7, 8, 11), amorphous polyamides (poly‐C‐methylcaprolactam, terpolymer of caprolactam, aminoheptanoic acid and aminoundecanoic acid) and model amides (N‐ethyl acetamide, Y‐ethyl butyramide, N‐butyl capronamide) at 27°C., with amides also at 40 and 55°C. On the basis of our own as well as reported data (on sorption isotherms, heat balance of sorption, sorption heats, infrared absorption, nuclear magnetic resonance, mechanical properties of polyamide‐water systems), we propose the following mechanism for the sorption of water in polyamides (capillary condensation is not considered): In nylon 6, three molecules of water are sorbed on two neighboring amide groups in an accessible region. The first molecule forms a double H‐bonds between the CO groups (firmly bound water) and the other two molecules form H‐bonds between the CO and the NH groups (loosely bound water). It turns out that even in nylons 2 to 6, water is probably sorbed by this mechanism whereas in nylons 8 to 12 only one molecule of water is sorbed on two amide groups, probably as a firmly bound water. Nylon 7 stands somewhere between the two groups.

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