Abstract: Ahighly sensitive method has been described for the determination of the trace molybdenum by the 2nd order derivative convolution voltammetry to determination of catalytic current in molybdenum-sulfur acid-mandelic acid-chlorate, in which the 2nd order derivative convolution, e', of the catalytic current,i, is measured as a function of electrode potential at a thin mercury film glass carbon electrode formed in situ. The peak height is directly proportional 10 the concentration of the molybdenum at the range of 10^-12-10^-8M. Samples of sea water, river water, well water and tapwater etc. can be determined directly without need of collection and separation.