Abstract Three component equilibria between H +, silicic acid, and the catecholamines dopamine and L-DOPA were studied by means of potentiometric (glass electrode) measurements in 0.6 M Na(Cl) medium at 25°C. Experimental data cover the ranges 2≤ −log[H +]≤9.5, 0.002≤0.005 M; 0.005 ≤ C ≤ 0.016 M, and 1 ≤ C/ B ≤ 6 ( B and C stand for the total concentration of Si and catecholamine, respectively). In acidic and near-neutral solutions no interactions were registered while, in slightly alkaline solutions, both systems are characterized by a formation of mononuclear tris-complexes SiL 3 containing pyrocatechol-type bonds. The species forming at −log[H +] ≈ 7.5 have fully protonated amine side-chains which, at somewhat higher −log[H +] values, start to deprotonate. Simultaneously, a decomposition of the octahedrally coordinated complex is occurring and the hydrolytic species SiO(OH) 3 − becomes increasingly important. With dopamine, the equilibrium constants characterizing the system are log K (Si(OH) 4 + 3H 3L + α Si(HL) 3 + + 2H + + 4H 2O) − 9.70 ± 0.03 and log K (Si(OH) 4 + 3H 3L + α SiL(HL) 2 + 3H + + 4H 2O) − 19.33 ± 0.03. The corresponding equilibrium reactions with L-DOPA are log K (Si(OH) 4 + 3H 3L α Si(HL) 3 2− + 2H + + 4H 2O) − 10.08 ± 0.05 and log K (Si(OH) 4 + 3H 3L α SiL(HL) 2 3− + 3H + + 4H 2O) −19.35 ± 0.07. Data were analyzed using the least-squares computer program LETAGROPVRID. The octahedral oxygen coordination around the silicon atom in these complexes has been verified by using the 29Si NMR method. Via model calculations it is shown that a minimum of ∼ 0.004 M catecholamine is needed to double the aqueous solubility of quartz.