To our knowledge, thin films of the primitive (𝑄 𝐼𝐼 𝑃 ) inverse bicontinuous cubic phase have not been reported. This phase is less commonly observed than the diamond (𝑄 𝐼𝐼 𝐷 ) or gyroid (QII G ) and typically appears in ternary lipid / surfactant / water mixtures, but it possesses unique structural features worthy of note: in particular, and unlike the 𝑄 𝐼𝐼 𝐺 and 𝑄 𝐼𝐼 𝐷 phases, it contains straight water channels, and therefore has been shown to be better able to accommodate linear particles such as siRNA; and the (100) facet, which we show in this work is the preferred orientation, presents a square array at the interface (p4mm). The (111) plane of the 𝑄 𝐼𝐼 𝐷 phase has hexagonal symmetry (p6mm) whilst the (110) plane of the 𝑄 𝐼𝐼 𝐺 phase has rhombic symmetry (c2mm).