Suspensions of palladium nanoparticles protected with poly(N-vinyl-2-pyrrolidone) (PVP) were prepared by refluxing the solution of palladium(II) chloride in methanol in the presence of PVP with various weight-average molecular weight (M-w = 6 000, 25 000, 175 000 and 574 000). The average diameter of palladium nanoparticles determined by transmission electron microscopy increased from 2.0 to 2.5 nm with increasing M-w, of PVP. The amount of PVP adsorbed on the palladium nanoparticles was determined by separation of palladium nanoparticle protected with PVP from dispersion medium using aminoethylated polyacrylamide gel. The amount of PVP adsorbed on palladium nanoparticles increased with increasing M-w, of PVP. The sedimentation coefficient of palladium nanoparticles was measured with ultracentrifugation for estimation of the thickness of adsorbed layer of PVP on palladium nanoparticles. The thickness of adsorbed layer of PVP on palladium nanoparticles increased from 1.9 to 7.8 nm with increasing M-w, of PVP. The higher molecular-weight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecular-weight fraction. The catalytic activity of the suspension of palladium nanoparticles protected with PVP was determined in the hydrogenation of 1,3-cyclooctadiene at 30 degrees C under 1 atm of hydrogen. The influence of adsorbed layer on the catalytic activity was investigated.