Water-soluble ytterbium complex of tetra-p-sulfonatophenylporphyrin (H2TPPS) with the formulation of Yb . TPPS . OH . 2 im . 3H(2)O (im = imidazole), stable in pH greater than or equal to 5 solution, has been synthesized and characterized by elemental analysis, IR and UV-visible spectra. The structural formula of this ytterbium porphyrin complex is proposed. The ytterbium atom is sitting above the porphyrin plane and has a coordination number of seven. The electrochemical behavior of the ytterbium complex in basic, neutral and slightly acidic solutions is also studied. The complex is strongly adsorbed at mercury electrode, and the center of adsorption, oxidation and reduction is the porphyrin ring. The mechanism of electrode processes for this ytterbium porphyrin complex has been investigated in detail.