Potassium salt of α,α′-bis(dithiocarboxyoxy)-p-xylene (abbreviated as K2-XDX) was synthesized as a chelating agent with p-xylene-α,α′-diol, carbon disulfide, and potassium hydroxide. Since a mercury(II)–XDX chelate complex suspension scattered pearl-blue light strongly because of the formation of a colloidal solution, the nephelometric analysis of small amounts of mercury(II) was undertaken. The calibration curve was found to be linear in the two ranges of 0.03–1.00 and 0.50–25.00 μg cm−3 of the mercury(II) ion respectively, but no suitable masking agents for interfering metal ions were found. The mole ratio of mercury(II) to XDX in the complex was found to be 1 to 1. These data for the complex suggested that XDX and mercury(II) combined alternately to form the (–Hg–XDX–)n complex. The molecular weight of this complex could be calculated as about 2×107 from the Zimm plot on the basis of the light-scattering method.