A phase diagram of the pseudoternary C12H25SO3Na + n-C4H9OH + n-C7H16 H2O system at k(m) = W(C4H9OH)/W(C12H25SO3Na)= 2 is presented and a continuous transparent one-phase microemulsion channel is obtained. An investigation of the structural transition in this one-phase microemulsion region is carried out by cyclic voltammetry and conductivity techniques. Both cyclic voltammetric and conductivity measurements show that the structural transition from a water in oil microemulsion into an oil-in-water microemulsion happens completely continuously without any phase separation, and it takes place in two stages: with increasing water content, first, an oil continuous microemulsion transforms into a bicontinuous structure at roughly 45% (wet%) water content, and then at roughly 65% water content a bicontinuous microemulsion turns into a water continuous structure.