The present paper describes a selective extractive spectrophotometric method for the determination of Co(II) using toluene solution of cyanex 923. A blue coloured complex (lambda(max) 685 nm) is quantitatively extracted in the organic phase from 7-10 mol L (HCl)-H-1. The composition of this complex is proposed as H2CoCl4.2cyanex 923. The Beer's law is obeyed in the concentration range 0.0- 4.2 x 10(-3) mol L-1 Co(II) and the molar absorptivity of the complex is 5.6 x 10(2) L mol(-1) cm(-1). The presence of hundred fold excess of metal ions such as Mo(VI), W(VI), V(V), Ti(IV), V(IV), Al(III), Cr(III), Fe(III), Mn(II), Fe(II), Ni(II), Cu(II), Zn(II) and Cd(II) and fifty fold of anions namely PO43-, SO42-, C2O42-, citrate, tartrate, NO3- and SCN- do not interfere in the determination of Co(II). The accuracy of proposed procedure is checked by assaying the concentration of cobalt in two standard NBS steel samples. The applicability of the method is demonstrated by selective determination of Co(II) in a multi element system like polymetallic manganese sea nodules. The result is in fairly good agreement with those obtained by AAS and ICP-AES.