Adsorbed layers of a diblock copolymer were characterized by measurements of hydrodynamic thickness. The polymer, poly(2-vinylpyridine)/polystyrene (PVP/PS), was adsorbed to porous mica membranes from toluene. A series of the diblock copolymer having a constant PS block (N-PS approximate to 430 monomers) was used to study the effects of the size of the anchoring group (PVP) on the thickness and coverage of the layer. The hydrodynamic thickness was measured in toluene (L(H)(T)) and in n-heptane (L(H)(dH)) after removal of the toluene by drying. The coverage to, chains per area on the pore wall) was estimated from L(H)(dH) by assuming both blocks collapsed into their bulk densities after being dried and after contact with heptane. Though there is a large scatter of our estimates of a, they are in reasonable agreement with the more accurate determinations of Parsonage et al. (Macromolecules 1991, 24, 1987). L(H)(T) increases at first with the size of the PVP block and then decreases when N-PVP/N-PS > 0.1. The maximum in L(H)(T) occurs at N-PVP/N-PS approximate to 0.05-0.10; our data are not extensive enough to precisely determine the position of the maximum. Using the data for a from Parsonage et al., we conclude that our results for the dependence of L(H) on N-PVP are not consistent with the scaling relation L(H) similar to N-PS sigma(1/3).