One important class of condensation polymers are polyamides. They arise from the reaction of carboxylic acid and an amine.  Examples include nylons and proteins. When prepared from amino-carboxylic acids, e.g. amino acids, the stoichiometry of the polymerization includes co-formation of water:
:n HN-X-COH  →  [HN-X-C(O)]  +  (n-1) HO
When prepared from diamines and dicarboxylic acids, e.g. the production of nylon 66, the polymerization produces two molecules of water per repeat unit:
:n HN-X-NH + n HOC-Y-COH  →  [HN-X-NHC(O)-Y-C(O)]  +  (2n-1) HO