In this work we discuss the oil chain length dependence of ternary DDAB microemulsions, comparing decane, dodecane, and tetradecane. With dodecane and shorter alkanes the L-2 microemulsion phase extends to the oil corner, while with tetradecane the microemulsion phase forms an island in the center of the ternary phase diagram. We present new NMR self-diffusion and N-14 NMR relaxation data, where the three systems are compared at similar compositions. It is argued that the disconnection of the microemulsion phase from the oil corner with longer oils is associated with the Winsor IT to Winsor III transition known from nonionic microemulsions. It follows that the tetradecane microemulsion has, in a major part, a monolayer rather than a bilayer structure.