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The chemical binders used to make sand molds and cores thermally decompose and release gas when subjected to the high temperature conditions in sand casting processes. Computational models that predict the evolution of the binder gas are being introduced into casting simulations in order to better predict and eliminate gas defects in metal castings. These models require knowledge of the evolved binder gas mass and molecular weight as a function of temperature, but available gas evolution data are limited. In the present study, the mass and molecular weight of gas evolved from PUNB bonded sand are measured as a function of temperature for use with binder gas models. Thermogravimetric analysis of bonded sand is employed to measure the binder gas mass evolution as a function of temperature for heating rates experienced in molds and cores during casting. The volume and pressure of gas evolved from bonded sand are measured as a function of temperature in a specially designed quartz manometer during heating and cooling in a furnace. The results from these experiments are combined with the ideal gas law to determine the binder gas molecular weight as a function of temperature. Thermogravimetric analysis reveals that the PUNB binder significantly decomposes when heated to elevated temperatures, and the PUNB binder gas mass evolution is not strongly influenced by heating rate. During heating of PUNB bonded sand at a rate of 2°C/min, the binder gas molecular weight rapidly decreases from 375 g/mol at 115°C to 99.8 g/mol at 200°C. The molecular weight is relatively constant until 270°C, after which it decreases to 47.7 g/mol at 550°C. The molecular weight then steeply decreases to 30.3 g/mol at 585°C and then steeply increases to 47.2 g/mol at 630°C, where it remains constant until 750°C. Above 750°C, the binder gas molecular weight gradually decreases to 33.3 g/mol at 898°C. The present measurements are consistent with the molecular weights calculated using the binder gas composition data from previous studies. The binder gas is composed of incondensable gases above 709°C, and the binder gas partially condenses during cooling at 165°C if the bonded sand is previously heated below 507°C.