For two polyphosphinoacrylates, PPAA-I and PPAA-II, and a random copolymer of maleic acid and vinyl sulfonic acid, PMA-PVS, the levels of adsorption equilibrium on BaSO4 crystals were determined as a function of their concentration and related to the formerly established growth-inhibiting effect of these compounds on BaSO4 crystallization. The best growth retarder of the two polyphosphinoacrylates, PPAA-II, reaches a higher surface coverage than PPAA-I which has a lower molecular weight. Such a relationship between inhibitor effect, maximal surface coverage, and molecular weight is only valid for structurally identical compounds. For the better growth retarder, PMA-PVS, a lower maximal surface coverage was found than for the structurally different PPAA-II. From the adsorption isotherms of PMA-PVA and PPAA-II, the equilibrium surface coverages at the PMA-PVS and PPAA-II concentrations which cause total growth blockage (0.2 ppm and 0.5 ppm, respectively) could be derived. This resulted in equilibrium surface coverages of approximate to 3.8 x 10(-3) mg m(-2) for PMA-PVS and approximate to 6.6 x 10(-3) mg m(-2) for PPAA-II. To determine whether matching between the inhibitor molecules and the crystal surface layer of BaSO4 plays a role in its performance, the shape and morphology of BaSO4 crystals were examined after long-term growth experiments in the presence of PMA-PVS and PPAA-II. The development of the {011} and {101} faces in the presence of PMA-PVS points at an easy replacement of the lattice sulfate ions with one S-O bond orientated perpendicular to the crystal surface by sulfonate ions of PMA-PVS. In the presence of PPAA-II, striations were observed in the [010] directions which might be related to a good fitting between the interatomic lattice distances in these directions and the mutual distances of the functional carboxylate groups of the inhibitor.