A neutron diffraction isotopic substitution experiment was carried out on the chloride ion in an aqueous solution of the polybase linear poly(ethylenimine) and DCl at high polymer charge density. The composite radial distribution function of all atomic species about Cl- (G(Cl)(r)) is compared with that of Cl- in a low molecular weight salt solution, and with that in a solution of the nonionic polymer poly(ethylene oxide). For the first time, it is observed that the presence of a linear polyion seriously affects G(Cl)(r) in an r-space region commonly referred to as the first hydration shell. This observation is suggestive of close contact between polyelectrolyte and counterion. The nuclear magnetic relaxation rate of Cl- is strongly enhanced in the presence of the polycation, and it is concluded that an increase in the coupling constant may at least be partly responsible for the observed NMR effect.