Highly porous composites were synthesized via facile crystallization of a metal-organic framework (MOF) in mesopores of a three-dimensional (3D) graphene (MG). First, MG was prepared via facile thermal exfoliation of graphite oxide in air, which enabled sufficient separation of graphene sheets in the form of an interconnected 3D structure. Next, aluminum-based MOF MOF520: Al8(OH)8(BTB)4(HCOO)4, BTB = 4,4′,4”-benzene-1,3,5-tryil-tribenzoate was predominantly crystallized in mesopores of 3D structure having a large pore volume. The resulting MG-MOF composites possessed specific surface areas and pore volumes between those of MG (640 m2/g and 3.0 cm3/g) and MOF (3160 m2/g and 1.3 cm3/g). The composite consisting of MG and MOF with a weight ratio of 1:2 showed almost twice-larger benzene adsorption capacity, i.e., 24.5 mmol/g at 20 °C and relative pressure close to unity, in comparison to that of pure MOF. Although at the same conditions the pure MG captured an ultrahigh amount of benzene, i.e., 33.6 mmol/g, adsorption of this adsorbate at low and moderate relative pressures was much lower than that on pure MOF. Thus, the MG-MOF composites featured much better benzene adsorption performance in a wide range of relative pressures as compared to pure MG and MOF, respectively. Additionally, the composites featured better thermal stability in comparison to the pristine MOF. The use of MG in this synthesis method afforded the well-dispersed, small crystals of MOF, which is essential for the design of MOF-based composites. Display omitted •Crystallization of MOF in mesopores of a three-dimensional graphene.•Enhancement of C6H6 adsorption by adding mesoporous graphene to MOF.•Ultrahigh C6H6 adsorption of 24.5 mmol/g on graphene-MOF composite at 20 °C.