The adsorption of hydrophobically modified polyelectrolytes (HMPEs) at hydrophobic surfaces was studied by ellipsometry. Both the adsorbed amount of polymer Gamma and the layer thickness d were measured as a function of time. The HMPEs are polyacrylates carrying various fractions of grafted dodecyl chains. Hydrophobic interactions are responsible for the adsorption (unmodified polyacrylate does not adsorb at the surface). The surface anchoring of grafted dodecyl groups is coupled with the repulsion of charged acrylate groups. This results in the formation of large loops at the interface, separated by very short trains, since the probability to have two or more successive alkyl groups grafted at the polymer chain is small. As in the case of ordinary polyelectrolytes, the mean thickness of the adsorbed layer decreases with increasing solution ionic strength. Changes of the HMPE chemical composition (molar ratio, X, of alkyl units attached on the polyacrylate backbone) affect the adsorbed layer structure in a complex fashion. HMPEs with relatively few grafted chains form thick dilute adsorbed layers. Increasing the ratio of grafted chains to the number of polyelectrolyte monomers results in a monotonic decrease of the thickness and an increase. of the HMPE density. However, the adsorbed amount exhibits a maximum at a relatively small ratio between 5% and 7%, above which the effect of the decrease of Gamma caused by the decrease of the average loop size becomes larger than the opposing effect of the increasing surface density of anchors. We also discuss the shape of the adsorption isotherms, experimental evidence for the existence of a depletion layer located outside the adsorbed layer, and the time evolution of adsorption and desorption.