A proton NMR study was carried out on complexes of Mn(II) with amino acids and peptides in aqueous solution. The theory, based on the Solomon-Bloembergen equations, was summarized and discussed. Different types of equilibria involving the anionic and zwittrionic forms of the amino acids were taken into account, together with the presence of outer- and inner-sphere species. The related correlation times were evaluated. Sarcosine, beta-alanine, threonine, tyrosine, valine, beta-alanine, proline, histidine and cystein complexes with Mn(II) were investigated on the basis of the NMR spectra. Moreover, diglycine and triglycine were studied in the presence of manganous ion at different pH value. The findings were discussed in terms of selective broadening of the NMR peaks and compared with data in the literature.