The reaction S_(2)O_(8)^(2-) + 3I^(-) rarr 2SO_(4)^(2-) + I_(3)^(-) is of first order both with respect to the persulphate and iodide ions. Taking the initial concentration as 'a' and 'b' respectively and taking x as the concentration of the trioxide at time t a differential rate equation can be written. Two suggested mechanisms for the reaction are: (I) S_(2)O_(8)^(2-)+I^(-) rarr SO_(4)I^(-)+SO_(4)^(2-)(fast) I^(-)+SO_(4)I^(-) overset(k_(1)) rarr I_(2) + SO_(4)^(2-)(slow) I^(-)+I_(2) overset (k_(2)) rarr I_(3)^(-)(fast) (II) S_(2)O_(8)^(2-)+I^(-) overset(k_(1)) rarr S_(2) O_(8) I^(3-)(slow) S_(2)O_(8)I^(3-) overset(k_(2)) rarr 2SO_(4)^(2-) +I^(+)(fast) I^(+)+I^(-) overset (k_(3)) rarr I_(2)(fast) I_(2)+I^(-) overset(k_(4)) rarr I_(3)^(-)(fast) For the reaction I_(2)+2S_(2)O_(3)^(2-) rarr S_(4)O_(6)^(2-)+ 2I^(-) (P) -(d[I_(2)])/(dt)=-1/2(d[S_(2)O_(3)^(2-)])/(dt) (Q) -(d[I_(2)])/(dt)=-2(d[S_(2)O_(3)^(2-)])/(dt) (R) (d[I^(-)])/(dt)=-2(d[I_(2)])/(dt)xx(d[S_(2)O_(3)^(2-)])/(dt) (S)(d[S_(4)O_(6)^(2-)])/(dt)=1/2(d[I^(-)])/(dt)