Abstract: A method and apparatus for measuring isotherms comprising a two-pressure and/or two-temperature humidity generator having a temperature controlled internal chamber and a microbalance mounted externally such that the mass of a sample suspended within the chamber can be determined without disturbing the controlled environment surrounding the sample. The two-pressure humidity generator has a front panel for controlling humidity and temperature within the internal chamber and 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 cheer. 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.