Porous liquids (PLs) combine the properties of permanent porosity with fluidity, offering a unique class of materials for selective gas capture and separation in solution. Here, we report a Type II PL formed by dissolving a shape-persistent, purely hydrocarbon molecular cage (1) featuring an intrinsic cavity in a sterically bulky, size-excluded solvent. Structural analysis, molecular dynamics (MD) simulations, and positron annihilation lifetime spectroscopy (PALS) unambiguously demonstrate effective solvent exclusion from the intrinsic cage cavity and preservation of permanent, cage-derived porosity in the liquid state. The hydrocarbon cage exhibits a strong physisorption affinity for CO₂ in the solid state (Q_st = 30–40 kJ mol^–1), arising from quadrupolar interactions under Å-scale confinement. As a proof-of-concept application illustrating how PLs can be tailored toward specific molecular separations, the resulting Type II PL was evaluated for ethylene/ethane separation, one of the most challenging hydrocarbon separations. The design of a purely hydrocarbon cage with a cavity shape and size matched to C₂ hydrocarbons, together with the identification of a compatible sterically bulky, size-excluded solvent, affords a molecularly sieving PL that selectively uptakes ethylene over ethane. The PL achieves iterative enrichment to ∼90% ethylene purity from an equimolar ethylene/ethane mixture through sequential adsorption–desorption cycles under mild thermal conditions. The study establishes purely hydrocarbon organic cages as modular building blocks for Type II PLs and demonstrates how intrinsic cage size and shape, and solvent exclusion can be rationally designed to adapt PLs to perform challenging hydrocarbon separations