A very strong particle charge dependence of the pressure independent flux has been demonstrated in the ultrafiltration of colloids. This flux passes through a minimum both as the charge of the particles increases, for constant particle size, and as the particle size increases, for constant particle charge. By considering the force balance on a particle and the flocculation of particles, the observed variation of experimental ultrafiltration flux have been modelled. The observed variation in flux with particle size at high particle charge is predicted. Variations in flux with particle charge are predicted for small particles, r(p) less-than-or-equal-to 25 nm, at low and high particle charge and for larger particles, r(p) greater-than-or-equal-to 100 nm, at high particle charge.