The periodic tricontinuous network structure (TNS) of symmetric isoprene (I)–styrene (S)–2-vinylpyridine (P) triblock copolymers has been studied by small-angle X-ray scattering (SAXS). The observed SAXS profiles were compared with the calculated diffraction intensities based on the model electron density distributions constructed for a pair of surfaces parallel to the infinite periodic minimal surface. In the model, an imaginary minimal surface exists at the centre of the styrene labyrinth and both I/S and S/P interfaces are on the parallel surfaces, powder orientation of the model structure being assumed. The experimental SAXS patterns for the samples with different styrene volume fractions,i.e.0.48 and 0.66, which are the lower and higher extremes for tricontinuous network structure, are well explained by the model calculation for surfaces parallel to the Gyroid minimal surface.