An isothermal equilibrium theory analysis of a simple two-step PSA process utilizing an adsorbate-adsorbent system that exhibits an unfavorable Langmuir isotherm has been carried out. Analytic expressions that directly describe the periodic state have been obtained and used to derive expressions for all of the important process performance indicators, with and without breakthrough of the heavy component into the light product. A design study with a H-2-metal hydride system revealed that the enrichment of this kind of PSA system ideally is always equal to the pressure ratio, regardless of whether breakthrough occurs. The breakthrough case compared to the no-breakthrough case caused the recovery of the light product to increase, but at the expense of the heavy-product recovery and light-product purity both decreasing below 100%. A parametric study revealed the effects of the most important process parameters on the process performance indicators and some subtle features that appear to be unique to this PSA process-isotherm combination. The design expressions developed here should be very useful for rapid feasibility studies, as the results represent the best possible separation that can be achieved with this kind of PSA process; they should also be insightful for training and educational purposes.