A　criterion　was　developed　to　determine　the　necessity　of　using　1D　or　2D　methods　for　modeling　a　concentric　tubular　membrane　reactor　for　hydrogen-producing　reactions.　Four　dimensionless　parameters　were　defined　to　provide　a　complete　description　of　the　physical　and　chemical　systems,　and　the　effects　of　each　parameter　on　the　difference　between　a　1D　and　2D　model　were　explored.　The　membrane　reactor　geometry　consisted　of　a　tube-and-shell　configuration　with　a　catalyst　bed　in　the　tubular　section　and　a　perm-selective　membrane　in　the　interphase.　The　four　dimensionless　parameters　consisted　of　the　Damk　&　ouml;hler　number　(Da),　which　relates　the　reaction　and　axial　convection　rates,　the　P　&　eacute;clet　number　(Pe),　which　relates　the　radial　diffusion　and　axial　convection　rates,　the　transport　parameter　(theta),　which　relates　the　axial　convection　and　membrane　permeation　rates,　and　the　equilibrium　parameter　(epsilon),　which　is　a　measure　of　the　favorability　of　the　reaction.　A　cutoff　at　which　1D　and　2D　model　results　have　less　than　a　1%　difference　in　final　concentrations　was　used　to　define　a　critical　Pe　value,　or　Pe(crit).　Then,　a　criterion　equation　was　fitted　using　non-linear　regression　so　that　any　system　can　be　calculated　and　compared　against　Pe(crit).　Using　this　criterion,　a　model　above　the　cutoff　(Pe　＞　Pe(crit))　can　reduce　computation　time　by　selectively　calculating　a　simple　1D　model　for　conditions,　while　a　model　below　the　cutoff　(Pe　＜=　Pe(crit))　should　use　a　more　computationally　expensive　2D　model　to　assure　accuracy.　The　final　criterion　was　validated　against　literature　data　for　both　1D　and　2D　models.