The phase behavior of long hydrophobic A-B type silicone surfactants, Me3SiO-(Me2SiO)(m-2)-Me-2-SiCH2CH2CH2-O-(CH2CH2O)(n)H (SimC3EOn), in water and water + octamethylcyclotetrasiloxane (D-4) was investigated by studying phase behavior and small-angle X-ray scattering. Si25C3EO15.8 forms a reverse micellar cubic phase (I-2) in water and water + D-4 systems. This cubic phase is highly thermally stable in a surfactant-water binary system. The thermal stability decreases monotonically with addition of silicone oil. Although the solubilization of water in the reverse cubic phase is low, a very large amount of excess water can be incorporated in a so-called reverse cubic phase based concentrated emulsion. The emulsion stability is enhanced upon addition of silicone oil. D-4 molecules penetrate into the surfactant palisade layer in the reverse micelles forming the I-2 phase and expand the effective cross-sectional area per surfactant, as (penetration). The continuous penetration of oil destabilizes the I-2 phase structure, and therefore the melting temperature of the phase decreases. The incorporation of D-4 into the I-2 phase in the aqueous mixtures of Si14C3EO7.8, Si25C3EO7.8, Si25C3EO12.2, and Si25C3EO15.8 varies with both the hydrophilic and lipophilic chain lengths of silicone surfactants.