A theory is developed for the surface coagulation of particles resulting from collisions between suspended flowing particles and particles deposited on a surface analogous to the classical Smoluchowski model for orthokinetic coagulation, with and without taking into account colloidal and hydrodynamic interactions, The theory predicts an increase in the capture frequency with an increase in the radial distance from the stagnation point, Experimental observations on the surface coagulation of calcium carbonate particles done in a well-defined hydrodynamic flow held created by an impinging jet are in agreement with theory for small radial distances but show a decrease in capture frequency for large radial distances, possibly due to surface roughness of the colliding particles.