The successful ATRP of a coordinating monomer, 4-vinylpyridine (4VP), in aqueous media at 30 C is reported. In the presence of basic and nucleophilic monomers such as 4VP, the use of the chloride-containing ATRP initiating/catalytic system is essential to achieve good polymerization control and narrow molecular weight distribution. This is due to the significantly slower reaction of the monomer or polymer with secondary alkyl chloride-type dormant chain ends compared to their alkyl bromide counterparts. When a bromide-based ATRP initiating/catalytic system was used, the obtained polymers had polymodal molecular weight distributions reflecting the formation of branched chains, whereas the polymers obtained with chloride-based initiating/catalytic system had narrow, monomodal, and symmetrical molecular weight distributions. The polymerization of 4VP mediated by the Cu-I complex of 2,2'-bipyridine was slow due to the low value of the ATRP equilibrium constant, in agreement with the low reducing power of the complex. Very good control over the polymerization was accomplished with the (CuCl)-Cl-I complexes of N, N, N', N'', N''', N'''-hexamethyltriethylenetetramine or tris(2-pyridylmethyl) amine (TPMA) as the ATRP catalysts. The TPMA complex is more reducing and therefore catalytically more active. In protic solvents, a significant part of the deactivating Cu-II halide complex dissociates and the radical deactivation becomes inefficient, leading to polymers with relatively high polydispersity. The use of a catalyst containing sufficient initial concentration of (CuCl2)-Cl-II complex (30% of the total Cu) improved the polymerization control.