This article describes an approach toward improving the characteristics of a photopolymer for holographic data storage application. The maximum diffraction efficiency (η max) and dynamic range (M#) of 9,10- phenanthrenequinone (PQ)-doped poly(methyl methacrylate; PMMA) both improved significantly after co-doping with one of three nitroanilines-N,N-dimethyl-4- nitroaniline (DMNA), N-methyl-4-nitroaniline (MNA), and 4-nitroaniline (pNA). In particular, the value of η max increased from 38% for the PMMA/PQ system to 72% for the PMMA/PQ/DMNA system (a 1.89-fold improvement) and the value of M# increased accordingly from 2.7 to 7.3 (a 2.70-fold improvement). Thus, the holographic data storage characteristics of PMMA/PQ photopolymers can be improved through co-doping with nitroaniline compounds. We also investigated the mechanism of the nitroaniline-induced improvement in optical storage performance using proton nuclear magnetic resonance and X-ray photoelectron spectroscopy.