Systematic investigation of the phase behavior of 16 surfactants with linear and branched hydrocarbon tails in oil and water mixtures reveals new relationships between surfactant structure and microemulsion formation. The surfactants examined are classified as “linear” (single-tail), “methyl branched”, “double tail”, or “highly branched”. Measurements of the phase behavior in oil and water clearly establish the lipophilic ranking: highly branched ≈ double tail > methyl branched > linear. In almost all cases, branched surfactants mix equal amounts of oil and water less efficiently than linear surfactants and phase behavior yields the efficiency ranking: linear > double tail ≫ methyl branched ≈ highly branched. Since branched ethoxylated alcohol/oil/water mixtures exhibit a larger three-phase region than linear ethoxylated alcohols, branched surfactant / oil / water mixtures are further from the tricritical point.