A simple condition for metallurgical instability useful in the development of processing maps for analyzing the high temperature forming of metals, is used. This condition is deduced following a criterion based on continuum principles as applied to large plastic flow proposed by Ziegler, which can be applied for any type of flow stress versus strain rate curve. This criterion has been validated using the flow stress data of a α2/O titanium aluminide alloy Ti-25Al-15Nb with microstructural observations. The optimum hot working conditions are suggested for this material based on the processing map.