ZSM-5 zeolite membranes with boron substituted into the framework for silicon were prepared on porous stainless-steel and cy-alumina tubular supports. These membranes had higher n-C4H10/i-C4H10 separation selectivities, and effectively separated these isomer mixtures to higher temperatures than membranes with aluminum substituted into the framework. Membranes were prepared with Si/B ratios as low as 12, and the best membranes were prepared from alkali-free gels. The highest n-C4H10/i-C4H10 permselectivity at 473 K was 60, and the highest at 527 K was 24. For most alkali-fr ee membranes, the n-C4H10/i-C4H10 permselectivities and separation selectivities increased with boron content, and membranes on alpha-alumina supports had both higher permeances and separation selectivities. Membranes with the same permeances and selectivities can be reproducibly prepared, and they are stable at elevated temperatures.