A response of a flexible rotor with a thermal bow caused by evaporation of water trapped in an axial bore is analyzed based on a case history in a vacuum balancing facility. Some vintage steam turbine rotors, after major factory repairs, require high speed balance at temperatures above 176 degrees C due to their material properties. During a routine heated high speed balance of an old HP/IP turbine rotor a very unusual behavior was experienced: response during run-down did not follow the response on run-up with very high vibration at the 1st critical speed. Such a behavior is indicative of a bow created in the rotor at operating speed. Investigation revealed that about 100 ml of water trapped in the axial bore was the cause of observed behavior. Thermodynamic modeling was performed to study the rotor's temperature distribution and explain why the water did not evaporate during initial heating to 176 degrees C. This case history provides a rare opportunity to isolate rotor's response due to thermal bow caused by trapped water evaporation in the absence of operational factors like rubs, coupled rotor influences, steam temperature and pressure, etc.