In the literature the transport of particles toward an oppositely charged collector in stagnation point flow is described theoretically with the Smoluchowski-Levich solution of the convective-diffusion equation. Using optical reflectometry, the initial adsorption rate in the case of adsorption of negatively charged polystyrene latex particles onto a glass collector covered with positively charged poly(propylene imine) dendrimers was studied at low ionic strength and constant pH, For latex particle sizes varying from 10 to 100 nm, the experimentally obtained values for the initial adsorption rate are systematically lower than the theoretical predictions, The deviations are captured by a sticking probability or collision efficiency that increases linearly from 0.5 to 0.8 within the experimental window, This can be explained by the fact that with particle size the contact area between the incoming particles and the amorphous collector increases and with this the number of sites involved in binding.