For the development of processing maps, a simple instability condition for assessing the extent of plastic deformation in a workpiece prior to the formation of defects, is derived based on the Ziegler's continuum principles. To demonstrate the potential of the instability condition, the published flow stress data of 6061 Al-10 vol.% Al2O3 particulate reinforced metal matrix composite is considered. Instability maps at different strain levels were superimposed while delineating the unstable regions in the processing maps. This takes into account the dependence of strain rate sensitivity and strain hardening coefficient of the material on the plastic instability during hot deformation. The stable and unstable regions in the map are verified with the microstructural observations of the deformed compression specimens as well as the industrial forging trials. To examine its validity, a comparative study is made with the flow localization concept for titanium alloys and titanium aluminides. The standard flow localization concept shows inconsistency in predicting the unstable regions in the processing maps, whereas, the present instability criterion is found to be consistent. Further studies were made on the hot deformation of 6061 Al with varying volume fractions and sizes of SiC particulate reinforcements. The 'stable' and 'unstable' regions in the processing maps identified from the present instability condition using the measured flow stress data, have been compared with the reported microstructural observations of the deformed compression specimens. It is noted from the processing maps that the domain of instability increases with the increasing volume fraction of the SiC particles. The optimum hot working conditions for these composites are suggested. © 2002 Elsevier Science Ltd. All rights reserved.