A promising candidate to fulfil the solid oxide fuel cell sealant requirements with respect to gas tightness, high temperature stability and electrical insulation appear to be glass-ceramic sealing materials. However, the reliable operation of solid oxide fuel cell stacks depends strongly on the structural integrity of the sealing materials. In this respect failure and deformation are aspects which need to be assessed in particular for glass ceramic sealant materials. Bending tests were carried at room temperature and typical stack operation temperature for glass ceramic sealants with different degree of crystallization. Elastic moduli, fracture stresses and viscosity values are reported. In addition to sintered bars bending testing were carried out for steel specimens that were head-to-head joined with the glass ceramics similar as in a stack application, where the ceramic particle reinforced sealant material was screen printed onto the steel. The results reveal a decrease of the strength for the partially crystallized sealant at operation relevant temperatures that can be associated with the viscous deformation. Fractographic analyses based on a combination of optical, confocal and scanning electron microscopy gave insight into the failure origin. (C) 2013 Elsevier B.V. All rights reserved.