Adsorbents and methods for the adsorptive separation of para-xylene from a mixture containing at least one other C.sub.8 aromatic hydrocarbon (e.g., a mixture of ortho-xylene, meta-xylene, para-xylene, and ethylbenzene) are described. Suitable adsorbents comprise nano-size zeolite X having an average crystallite size of less than about 500 nanometers. The adsorbents provide both improved capacity and mass transfer, which is especially advantageous for improving productivity in low temperature, low cycle time adsorptive separation operations in a simulated moving bed mode.