The report discusses the development of radon diagnostic procedures and mitigation strategies applicable to a variety of large non-residential buildings commonly found in Florida. The investigations document and evaluate the nature of radon occurrence and entry mechanisms for radon, the effects of heating, ventilation, and air-conditioning (HVAC) system configuration and operation on radon entry and dilution, and the significance of occupancy patterns, building height, and other building construction features. A primary focus of the project was the effect of the HVAC systems of a large building on the transport, entry and (hopefully) the minimization of indoor radon in the building. Two buildings were investigated, both of which showed an inverse relationship between dedicated ventilation air and indoor radon concentrations, as was expected. Both also showed signs of unusual HVAC design, operation, and maintenance that presumably adversely affected indoor radon and other indoor air quality (IAQ) variables.