Small-angle X-ray scattering (SAXS), circular dichroism (CD), and density measurements were made on eight samples of collagen model peptides consisting of glycine (Gly), proline (Pro), and 4-(R)-hydroxyproline (Hyp), i.e., (Gly-Hyp-Hyp)(n), (Gly-Pro-Hyp)(n), (Gly-Hyp-Pro)(n), and (Gly-Pro-Pro)(n) (n = 5 and 9) in pure water and in phosphate buffered saline at a wide range of temperatures between 4 and 75 degrees C. A complete triple-helical structure was found only for (Gly-Hyp-Hyp)(9) and (Gly-Pro-Hyp)(9) at 15 degrees C by SAXS and CD, and (Gly-Pro-Pro)(9) had a partially formed triple helix at 4 C-degrees. At 70 degrees C or higher temperatures, all peptides are dispersed as monomeric chains in these solvents and have essentially the same radii of gyration as the literature values for denatured proteins with the equivalent number of amino acid residues, suggesting that the conformational entropy for the single chain of the collagen model peptides is almost independent of the imino acid content. Furthermore, the partial specific volume (v) over bar for (Gly-Hyp-Hyp)(9) around the transition temperature decreases more gradually with decreasing temperature than that for (Gly-Hyp-HYP)(5). On the contrary, the temperature dependence of (v) over bar for (Gly-Pro-Hyp)(n) has the opposite tendency, indicating that the hydration number for the peptide (Gly-Pro-Hyp)(9) increases with formation of the triple helix whereas that for the peptide (Gly-Hyp-Hyp)(9) decreases. These results are consistent with the difference in the transition enthalpies reported previously [J. Biol. Chem. 2004, 279, 38072].