Abstract: A high temperature humidity determining device which measures the moisture content of gas streams with temperatures as high as, but not limited to 600.degree. F. The device uses a solid state, thin film polymer type relative humidity/temperature sensor and electronic barometer to measure the properties of the gas stream. The unit does not require laboratory quality oxygen, ice water, distilled water or any other cleaning fluids. In order to avoid damage to the sensor, a sample is cooled to below about 300.degree. F. prior to sampling. This is done by pumping the sample air through a thermostatically controlled sample conduit, and into a positively pressurized and thermostatically controlled sample chamber. A microprocessor calculates the humidity ratio and dew point of the sample or other moist air properties, and delivers data to a visual process indicator, as well as to any other desired output.