The use of ice and chilled water storage systems to reduce peak energy demands is well established. However, relatively few experimental results have been published on the use of building thermal mass to offset peak energy demand. The use of off peak cooling to extract heat from a buildings thermal store may increase energy use while reducing peak cooling loads and cooling costs. This paper describes a project sponsored by ASHRAE TC 4.6 on building thermal storage. The purpose of this research project was to evaluate the effect of building thermal energy storage on the peak cooling load. Two experiments were performed on the Independent Life Insurance building located in Jacksonville Florida. This building is 37 stories high with one million square feet of floor area. The objective of the experiments was to pre-cool the building at night and during the weekend to reduce daytime cooling loads. Measurements of airflow and temperature on a story with pre-cooling and a story without pre-cooling were used to calculate the energy supplied to each story. Additional measurements of temperature and heat flux at the concrete floor surface were made on the pre-cooled story Results from the experiment show the reduction in cooling load due to pre-cooling. Temperature and heat flux measurements at the concrete floor surface reveal the extent of charging and discharging of the thermal mass. Results from the experiments were compared to diurnal heat capacity calculations. Diurnal heat capacity is a frequency method which can be used to calculate the potential for energy storage in a building.