An operability level coefficient (OLC), defined as the ratio of product permeation and product formation rates, and related to the inverse combination of the Damkohler number and the Peclet number (1/DaPe), is suggested as a useful tool for estimating performances of membrane reactors (MRs) operating as separators in equilibrium-limited reactions. The OLCs for product hydrogen formation in previously reported MRs for methane dry-reforming (MDR), methane steam-reforming (MSR), methanol steam-reforming (MeOHSR), and ethanol steam-reforming (EtOHSR) were correlated with conversion and yield enhancements. For values of OLCs ranging from 0.03 to 0.78, a clear universal trend for increasing conversions and hydrogen yields with increasing OLC was observed for these different types of reforming reactions. The OLC curve calculated from a numerical simulation without adjustable parameters was found to closely approximate experimental data obtained from the MRs, and was shown not to depend on the assumed kinetics. This study confirms that hydrogen selectivity (from the ratio of single-gas permeances) has a substantial influence on conversion and hydrogen yield enhancements in a MR, and demonstrates that a hydrogen selectivity of 100 is sufficient to achieve high performance in a MR. (C) 2009 Elsevier B.V. All rights reserved.