The impinging jet is a well-established technique to study the deposition of colloidal particles onto a solid surface (a collector) under well-defined hydrodynamic and physicochemical conditions. The presence of polymers, in solution, adsorbed onto the colloidal particles or onto the collector surface, can have a large effect on the deposition kinetics of the particles. In this study we show that it Is possible to characterize polymers by using colloidal particles as probes. By choosing experimental conditions under which a competition takes place between polymer adsorption and particle deposition on the solid surface, the kinetics of particle deposition is governed by the properties of the polymers. Under conditions for which particle deposition is absent when both the particles and the solid surface are coated by polymer, the impingement of a mixture of polymers and polymer-coated particles will cause an initial deposition of polymer-coated particles which stops when the solid surface is coated by polymers in solution. The time at which particle deposition stops equals the time tau(ads) at which the solid surface is coated by polymers. This time depends on the polymer concentration, c, and molecular weight, M. Thus, in principle, measuring tau(ads) yields information about c and/or M. Here we show that adsorption isotherms can be obtained for a polyelectrolyte (polyethylenimine) adsorbing on TiO2 particles, by measuring c with the impinging jet technique. The obtained isotherm closely resembles the one obtained by classical methods (colloid titration), The advantage of the present method is that the technique can be applied for very low polymer concentrations, typically in the range 10 ppb to 1 ppm.