Synthetic and natural polymers are often used as drug delivery systems in vitro and in vivo. Biodegradable chitosan of different sizes were used to encapsulate antitumor drug tamoxifen (Tam) and its metabolites 4-hydroxytamoxifen (4-Hydroxytam) and endoxifen (Endox). The interactions of tamoxifen and its metabolites with chitosan 15, 100 and 200 KD were investigated in aqueous solution, using FTIR, fluorescence spectroscopic methods and molecular modeling. The structural analysis showed that tamoxifen and its metabolites bind chitosan via both hydrophilic and hydrophobic contacts with overall binding constants of K(tam-ch-15) = 8.7 ( ± 0.5) × 10(3) M(-1), K(tam-ch-100) = 5.9 (± 0.4) × 10(5) M(-1), K(tam-ch-200) = 2.4 (± 0.4) × 10(5) M(-1) and K(hydroxytam-ch-15) = 2.6(± 0.3) × 10(4) M(-1), K(hydroxytam - ch-100) = 5.2 ( ± 0.7) × 10(6) M(-1) and K(hydroxytam-ch-200) = 5.1 (± 0.5) × 10(5) M(-1), K(endox-ch-15) = 4.1 (± 0.4) × 10(3) M(-1), K(endox-ch-100) = 1.2 (± 0.3) × 10(6) M(-1) and K(endox-ch-200) = 4.7 (± 0.5) × 10(5) M(-1) with the number of drug molecules bound per chitosan (n) 2.8 to 0.5. The order of binding is ch-100>200>15 KD with stronger complexes formed with 4-hydroxytamoxifen than tamoxifen and endoxifen. The molecular modeling showed the participation of polymer charged NH2 residues with drug OH and NH2 groups in the drug-polymer adducts. The free binding energies of -3.46 kcal/mol for tamoxifen, -3.54 kcal/mol for 4-hydroxytamoxifen and -3.47 kcal/mol for endoxifen were estimated for these drug-polymer complexes. The results show chitosan 100 KD is stronger carrier for drug delivery than chitosan-15 and chitosan-200 KD.