An approach for multibit-per-site optical data storage has been proposed and demonstrated using mesostructured composite films incorporated with uniformly dispersed photoacid generator and pH-sensitive dye molecules. Upon light illumination, photoacid generator molecules produce acid, which induces a change in the absorption property of pH-sensitive dye molecules. Because the amount of the generated acid is proportional to the illumination time, the resulting change in the absorption property of mesostructured composite films varies as a function of the illumination time. This function between the absorption property of mesostructured composite films and the illumination time can be used for multibit-per-site optical data storage. Recording is performed by using certain discrete values of the illumination time to represent information bits. Reading out is achieved by measuring the absorbance of composite films at a particular wavelength, from which the stored information bits can be determined. In general, N-bit-per-site storage can be realized using 2N discrete values of the illumination time. This multibit-per-site approach for optical data storage is compatible with the current single-bit-per-site technology used for compact disks and digital versatile disks, and will provide significantly larger optical storage capacity. It is also suitable for two-photon multilayer optical data storage if the photoacid generators and pH-sensitive dyes are properly designed.