A sealing agent for ion transport membranes (ITMs) includes a composition having a glass powder and a ceramic powder. The ceramic powder can include Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) or La2NiO4+δ (LNO). The ceramic powder can be identical to the ceramic powder from which the ITM is made. The glass powder can include PYREX glass. The sealing agent can be in the form of a paste. The sealing agent can be used to attach an ion transport membrane to one or more support tubes. The sealing agent includes from about 10 wt. % to about 40 wt. % glass powder and from about 60 wt. % to about 90% wt. % (BSCF) ceramic powder.