Propane-propylene separation is usually done by cryogenic distillation, one of the most energy consuming processes in the chemical industry. Alternative technologies are simulated moving bed (SMB) and vacuum swing adsorption (VSA). In the context of propane-propylene separation by SMB, the choice of suitable desorbents is a key issue. Recent experimental studies on isobutane and 1-butene as desorbents to operate a propane-propylene separation with an SMB, using zeolite 13X as solid adsorbent, motivated the present work. In this paper, the single component adsorption isotherms of isobutane and 1-butene and X-Y diagrams of binary mixtures of the desorbents with propane and propylene in zeolite 13X are computed using a configurational-bias Monte Carlo (CBMC) technique for a temperature range from 333 to 393 K and pressures up to 150 kPa. The interactions between the tertiary sp(3) carbon (CH), the sodium cations, and all other atoms of the studied systems have been determined, since no data for these interactions are available in the literature. All other parameters for alkane-alkane, alkane-sodium, and alkane-zeolite interactions were taken from a recently developed force field of Calero et al. (J. Am. Chem. Soc. 2004, 126, 11377) for Na-Faujasites. Also, we report molecular simulation results of 1-butene adsorption in zeolite 13X. The interactions of sp(2)-bonded carbons were taken from our previous work. The result of this work is a new set of force field parameters, extended for branched alkanes, that successfully reproduces experimental data that can be used to guide further development of SMB processes.