Scale inhibitors are widely used to prevent mineral scale deposition and precipitation in industrial processes. In this study, the performance of scale inhibitors is systematically investigated in a flow loop apparatus, designed to mimic the barite deposition in pipelines. The inhibitors are tested in tubing with and without a precovered barite scale layer. DTPMP (diethylenetriamine-pentamethylene phosphonic acid), PPCA (phosphinopoly carboxylic acid), and SPCA (sulfonated poly(carboxylic acid)) are the main inhibitors tested in this work. The heterogeneous deposition rate constants at 120 degrees C in a clean tubing in the presence of 0.25 ppm of DTPMP/PPCA/SPCA are 0.00, 10.3 x 10(-5), and 32.1 x 10(-5) cm/s, respectively. The deposition rate constants at 120 degrees C in a tubing fully covered with barite in the presence of 2.0 ppm of DTPMP, PPCA, or SPCA are 5.97 x 10(-5), 36.8 x 10(-5), and 28.0 x 10(-5) cm/s, respectively. In tubing fully covered with barite, all three inhibitors require a higher concentration to reach the same level of inhibition as observed in the uncoated tubing. The experiments in fully covered tubing were conducted in a wide temperature range (50-150 degrees C) with various dosages of inhibitors (0.02-40 ppm). The impact of inhibitors on deposition rate constants is modeled with a Langmuir-type equation by using an adsorption equilibrium constant K-eq, fractional occupancy theta, and unoccupiable fraction theta(0) in the fully covered tubing. theta represents the fraction of active sites occupied by the inhibitors, and theta(0) represents the fraction that cannot be occupied by the inhibitors. The adsorption isosteric enthalpies of functional groups on the inhibitor molecules are calculated from the fitted associate equilibrium constants. A kink site adsorption mechanism has been proposed as the inhibition mechanism.