Glass-ceramics of the BaO-CaO-Al2O3-SiO2 system are frequently used in planar solid oxide fuel cells (SOFC) stacks to seal the fuel and air compartments and to join non-conductively the individual components. Due to the thermal mismatch of the ceramic and metallic materials in the stack, the seals experience predominantly shear stresses. A symmetric shear test has been developed to characterize the critical shear stress of the glass-ceramic at SOFC operation temperature. Specimens representative for the seal situation in an SOFC stack were prepared, using the glass-ceramic to join a center piece of a NiO-YSZ anode covered by yttria-stabilized zirconia (YSZ) electrolyte layers on both surfaces between two Crofer22APU interconnect steel blocks. Shear stress and based on a rheological model, shear modulus and viscosity of the sealant were determined. The investigations showed that the sealant exhibits viscous shear deformation at 800 degrees C, a temperature typical for SOFC operation. The influence of increasing crystallization on the shear deformation is demonstrated.