Abstract: The reaction mechanism of detection methods of SO₂ with formaldehyde absorbing-PRA spectrophotometry and NH₃ with Nessler reagent spectrophotometry is reviewed. There are two steps in the reaction of SO₂ absorbed by formaldehyde buffer solution:SO₂ dissolves in the water to form H₂SO₃ firstly and then H₂SO₃ reacts with HCHO to form HO-CH₂-SO₃H. It involves both nucleophilic and electrophilic addition. The color reaction involves diazo reaction mechanism. The interference of NO_x is eliminated while H₂SO₃ reacts with HCHO to form diazonium ion. The pale-yellow complexes generated by reaction of nessler's reagent with ammonia are the dinuclear complex in which atoms of O and N act as intermediate base connecting two Hg center ion. Therefore, it is needed to control the proportion of materials to generateHgI₄^2- while preparing the Nessler reagent.