In our previous papers (Langmuir 1994, 10, 4423; 1996, 12, 5588; 1999, 15, 573), we have reported the salt concentration dependence of interparticle distance in colloidal crystals formed in highly charged colloidal dispersion by performing ultra-small-angle X-ray scattering (USAXS). We found that the interparticle distance-salt concentration relationship had a maximum at kappa a = 1.3 (kappa(-1), Debye length; a, particle radius). This result could not be explained by current theories proposed for colloidal stability, such as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. From the results of our previous UAXS study, we thought that the interpenetration of ionic clouds around particles and counterion sharing effect play an important role for the formation of colloidal crystal. In this study, we have investigated the added NaOH dependence, i.e., the degree of neutralization dependence of strong acid group at particle surface (a), on the nearest interparticle distance (2D(exp)) in colloidal crystals by the USAXS technique. The added NaOH dependence was found to be divided into four regions. In the first region at low degree of neutralization (0 < a < 0.1), 2D(exp) increased with increasing a and the particles formed a solidlike structure. In the second region (0.1 < a < 1.0), 2D(exp) showed saturation. In the third region (1.0 < a < 2.2), 2D(exp) decreased with increasing a and the particles formed a liquidlike structure. The most important finding in this study is the trend of 2D(exp) at 0 < a < 1.0. In this region, the counterion H+ of strong acid group at the surface of the particle is replaced by Na+ with increasing a. The fact that the change of 2D(exp) was observed in this region reflects the affect of counterion species on the interparticle "electric" interaction. It can be thought that the counterion sharing by extraordinary overlap of counterion clouds and the unique character of the H+ ion are significant and dominant factors for the colloidal crystal formation.