Conformations of partially or fully adsorbed semiflexible polymer chains are studied varying both contour length L, chain stiffness, κ, and the strength of the adsorption potential over a wide range. Molecular Dynamics simulations show that partially adsorbed chains (with “tails”, surface attached “trains” and “loops”) are not described by the Kratky-Porod wormlike chain model. The crossover of the persistence length from its three-dimensional value (lp ) to the enhanced value in two dimensions (2lp ) is analyzed, and excluded volume effects are identified for L ≫ lp . Consequences for the interpretation of experiments are suggested. We verify the prediction that the adsorption threshold scales as lp−1/3.