Ionic residues influence the stability of collagen triple helices and play a relevant role in the spontaneous aggregation of fibrillar collagens. Collagen types I and II and some of their CNBr peptides were chemically modified in mild conditions with two different protocols. Primary amino groups of Lys and Hyl were N-methylated by formaldehyde in reducing conditions or N-acetylated by sulfosuccinimidyl acetate. The positive charge of amino groups at physiological pH was maintained after the former modification, whereas it was lost after the latter. These chemical derivatizations did not significantly alter the stability of the triple helical conformation of peptide trimeric species. Also the enthalpic change on denaturation was largely unaffected by derivatizations. This implies that no significant variation of weak bonds, either in number or overall strength, and of entropy occur on modification. These properties can probably be explained by the fact that chemically modified groups maintain the ability to form hydrogen bonds.