Brownian dynamics (BD) simulations are reported for monovalent counterions about the parent macroion for different values of the surface charge Z(p) and the radius a(p). The counterion distribution functions g(pc)(r) thus obtained were used to determine a "thermal radius" r(therm) defined by the condition g(pc)(r(therm)) = exp(1), viz., when the interaction energy of the counterion with the parent macroion is equal to the thermal energy k(B)T. An "effective charge" Z(eff) was thus obtained by including with the bare charge Z(p) the equilibrium distribution of counterions that lie within the distance r(therm). These data, represented as Z(eff)/Z(p) versus Z(p)/a(p), are shown to follow the trend in the experimental data summarized by Roberts, O'Dea, and Osteryoung (Anal. Chem., 1998, 70, 3667). On the basis of the shape of this plot, three macroion classifications were indicated: (1) a steep initial, slope for Z(p)/a(p) < 30 nm(-1) (category I); (2) a "transition" region 30 nm(-1) < Z(p)/a(p) < 90 nm(-1) with a variable slope (category II); and (3) a shallow terminal slope for Z(p)/a(p) > 90 nm(-1) (category III). The dynamics of the counterions in each category was inferred from the ratio Delta r/\Delta r\, where Delta r is the difference in the radial displacement of the counterion at the initial and final positions and \Delta r\ is the magnitude of the vector difference between these locations. It was thus shown that the counterions in categories I and II are highly mobile whereas they are somewhat restricted to the vicinity of the macroion surface in category III macroions. These results are compared with theories of "charge renormalization" in the literature, and implications of the current model are discussed.