Abstarct: This work reports a simple and sensitive on-line solid phase extraction system for preconcentration and determination of trace amount of Co (II (by flame atomic absorption spectrometry (FAAS (. The method is baxsed on the on-line retention of cobalt at pH=6.5 with poly vinyl chloride modified by 3-(2-thiazolylazo)-2,6-diaminopyridine (TADAP). The retained Co (II) was eluted with 1.0 mol L-1 HCL and introduced directly in to the nebulizer of AAS for its determination. The influence of chemical and flow variables on the senstivity were optimized for the quantitative preconcentration of cobalt (II) and the interference of various ions on the on-line determination of Co (II) was also stadied. At optimized conditions, the calibration graph was obtained linear over the concentration rang of 2-150 µg L-1, for preconcentration of 10.0 ml sample solution. The relative standard deviation (n=6) at 0.01, 0.05, 0.10 and 0.14 mg L-1 level for cobalt were 5.2%, 3.0%, 1.8% and 1.8%, respectively. The 3