Trace amount of cobalt in molecular sieve was determined by simple differentialoscillographic voltammtry in Co-PAR system (PAR= 4-(2-pyridylazo) resorcinol). Resultsshowed that in a solution of 0. 2 mol/L HAc-0. 2 mol/L NaAc-2. 5 ×10-5 mol/L PAR (pH=4. 7) cobalt was able to cause a sharp incision on the cathodeic branch of the oscillogram at- 1. 68 V(vs. SCE). The method is simple, rapid and has less interferences than spectropho-tometry.