A method and apparatus for measuring isotherms and determining solid state phase changes comprising a humidity generator having a temperature controlled internal chamber, a microbalance mounted externally to the internal chamber and a near infrared spectroscopy (NIRS) probe or Raman spectrometer probe mounted internally in the internal chamber, such that the mass of a sample suspended within the chamber and its solid state phase can be determined without disturbing the controlled environment surrounding the sample. The humidity generator includes means for controlling humidity and temperature within the internal chamber, an access port that extends into the internal chamber. The balance is situated near the port such that the balance mechanism extends within the internal chamber. The balance mechanism utilizes a means for holding a sample within the chamber such that the mass of the sample can be determined as it equilibrates to the temperature and relative humidity maintained within the internal cavity. The NIRS probe extends from a NIR spectrometer. Similarly, the Raman probe extends from a Raman spectrometer.