A synergistic simple and selective spectrophotometric method was developed for the determination of cobalt(II) with 1-(2',4'-dinitro aminophenyl)-4,4,6-trimethyl-1,4-dihydropyrimidine-2-thiol [2',4'-dinitro APTPT] as a chromogenic reagent. The proposed method has been described on the basis of synergistic effective extraction of cobalt(II) in presence of pyridine at pH range 9.5-10.2, showed orange-red coloured ternary complex having molar ratio 1:2:2 (M:L:Py). The equilibrium time is 10 min for extraction of cobalt(III) from organic phase. The absorbance of coloured organic layer in chloroform is measured spectrophotometrically at 490 nm against reagent blank. The Beer's law was obeyed in the concentration range 2.5-15 μg mL(-1) of cobalt(II) and optimum concentration range was 5-12.5 μg mL(-1) of cobalt(II) and it was evaluated from Ringbom's plot. The molar absorptivity and Sandell's sensitivity of cobalt(II)-2',4'-dinitro APTPT-pyridine complex in chloroform are 1.109×10(3) L mol(-1) cm(-1) and 0.053 μg cm(-2), respectively while molar absorptivity and Sandell's sensitivity of cobalt(II)-2',4'-dinitro APTPT complex in chloroform are 6.22×10(2) L mol(-1) cm(-1) and 0.096 μg cm(-2), respectively. The composition of cobalt(II)-2',4'-dinitro APTPT-pyridine complex (1:2:2) was established by slope ratio method, mole ratio method and Job's method of continuous variation. The ternary complex was stable for more than 48 h. The interfering effects of various cations and anions were also studied, and use of suitable masking agents enhances the selectivity of the method. The method is successfully applied for the determination of cobalt(II) in binary, synthetic mixtures and real samples. A repetition of the method was checked by finding relative standard deviation (R.S.D.) for n=5 which was 0.15%. The reliability of the method is confirmed by comparison of experimental results with atomic absorption spectrophotometer.