An on-going investigation of soot deposition that began in 2014 is being conducted at the Vanderbilt Mansion by the Historic Architecture, Conservation and Engineering Center (HACE), of the National Park Service and EYP Architects. Interior surfaces, furnishing and historic objects in the mansion have been chronically subjected to soot deposition for the past 120 years. Though the soot may be entering the supply ductwork, and then the historically furnished rooms, the placement of furnace filters over the registers had been previously employed to mitigate the effect. Since it has not, it was determined that a more detailed examination of the soot, heating system and air circulation in the Vanderbilt Mansion should be performed. Initial efforts of the investigation included an assessment of heating system configurations, the collection of soot samples throughout the mansion, running X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Portable X-ray fluorescence analysis (pXRF), and Infrared Spectroscopy analysis of the samples in order to determine their source. Designed by McKim, Mead and White and built in the late 1890’s by Frederick and Louise Vanderbilt in upstate New York, the Vanderbilt Mansion was the height of opulence and Gilded Age luxury. Mrs. Vanderbilt’s Louis XIV bedroom, with expertly painted and gilt walls designed by Ogden Codman Jr., and Frederick’s masculine Renaissance Revival bedroom with its dark wood and tapestry covered walls still retain an exceptional degree of integrity to the Vanderbilt period and original construction. The Vanderbilts occupied the mansion during the summer and early fall months, occasionally staying over a weekend in the winter months. During this time, the mansion was heated by convection- based warm air system utilizing a fully ducted supply air distribution system driven by two coal-fired steam boilers located in the sub-basement of the north side of the building. When the National Park Service received the property from Mrs. Vanderbilt’s niece, in 1939, several upgrades were made to the property, including the installation of new coal fired steam boilers that were eventually retrofitted with oil burners in 1950. In the early 1970s, a portion of the convection system was reconfigured as a forced air system utilizing an air handling unit and the existing boilers. It was originally believed that this system produced the large amounts of soot that can be seen on the historic fabric in the mansion today due to poor combustion draft conditions. Consequently, in 2004 new oil boilers were installed to replace the coal fired boilers. However, large amounts of soot continue to be deposited throughout the mansion. The study thus far has hypothesized that the soot may circulating throughout the mansion directly from the subbasement level. More testing, including Differential Pressure Monitoring will be conducted this fall in order to either confirm or refute this hypothesis.