A Sensitive method for determination of Cobalt With tetra-(P-trimethylammo-niumphenyl) porphine iodide (TAPP) has been developed. Cobalt reacts With TAPP form a 1:1 Complex at pH 9.5, The sensitivity has been raised remartly by adding a Certain ethylenediamine at this pH after the Cobalt Complex has been formed. Apparent absorption Coefficient of Cobalt Complex is 2.78×10,5,.