The flow stress as a function of strain was measured in the temperature range from 77 to 400°K at strain rates between 10−1and 10−6sec−1for single crystals of molybdenum. The flow stress is characterized by a rate‐temperature parameter, defined through a general rate equation for low temperature thermally activated deformation processes in bcc structures. This parameter was utilized for testing the mechanical equation of state and in explaining the failure of such an equation in molybdenum. Whereas the rate of work hardening was found to be uniquely defined by strain and the parameter, the flow stress depended in addition on the thermal‐mechanical history of the material.