Large-pore phenyl-bridged periodic mesoporous organosilicas (PMOs) were facilely synthesized by evaporation-induced self-assembly of 1,4-bis(triethoxysily)benzene and triblock copolymer Pluronic F127 as a template under acid conditions combined with a mixed-solvothermal treatment. The ordered PMOs exhibit large uniform mesopores of ∼9.9 nm in diameter after calcination at 350 °C in a nitrogen atmosphere. The mesoporous phenyl-bridged organosilica products have an ordered hexagonal mesostructure with space group p6mm. N2 adsorption/desorption isotherms reveal imperfect mesopore channels with high surface areas (up to 1150 m2/g) and thick pore walls (up to 7.7 nm). The mesopores can be expanded with the decrease of acidity, as well as the increase of Pluronic F127 content. A mixed-solvothermal treatment in N,N-dimethylformamide (DMF) and water at 100 °C was first used to improve the periodicity of the mesopore walls, as well as increase the wall thickness. The composites exhibit efficient adsorption capacities (2.06 mmol g−1) for benzene, suggesting a potential adsorbent for removal of volatile organic compounds. The EISA approach combined with the mixed-solvothermal treatment provides important insights into the development of large-pore PMOs by using long-chain organosilanes, and further demonstrates the ability to fabricate materials with thick walls.