The triblock copolymer Pluronic P-123 (P123) templated silica films were deposited by spin coating on p-type silicon (100) wafers. Trimethylchlorosilane (TMCS) was added to precursor silica sols to provide the necessary hydrophobicity for the resulting silica films. The dielectric constant and leakage current density of in situ derivatized mesoporous silica films were found to decrease with increasing concentration of TMCS in precursor solutions. The precursor aging time was found crucial on the surface morphology and reliability in dielectric properties of mesoporous silica films with ultralow dielectric constants of 1.7-2.1.