AbstractPoly(N‐vinyl‐2‐pyrrolidone) (PVP) was chosen as a protecting polymer for its large protective value among homopolymers. Suspensions of palladium (Pd) nanoparticles were prepared by refluxing solutions of palladium(II) chloride and PVP in methanol. The mean diameter of Pd nanoparticles without adsorbed layer of PVP was controlled in the range 1.1–2.5 nm. The concentration of free PVP in the methanol suspension was determined by using aminoethylated poly(acrylamide) gel beads which adsorbed selectively the PVP‐protected Pd nanoparticles. The amount of PVP adsorbed on Pd nanoparticles increased linearly with the 0.45 power of weight‐averaged molecular weight (Mw) of PVP. The thickness of adsorbed layer of PVP was estimated from the sedimentation coefficient of the PVP‐protected Pd nanoparticles. The adsorbed layer thickness increased linearly with the 0.55 power of Mw of PVP. The radius gyration of free PVP in methanol increased linearly also with the 0.55 power of Mw of PVP. The catalytic activity of the suspension of Pd nanoparticles was determined in the hydrogenation of 1,3‐cyclooctadiene. The activity depended more effectively on the specific surface area of Pd nanoparticles than the adsorbed layer thickness. On the basis of these results, the conformation of protecting PVP molecule on the surface of Pd nanoparticles was proposed. Copyright © 2001 John Wiley & Sons, Ltd.