Potential reduced exposure products (PREPs) purport to lower toxicant emissions, but without clinical and long-term health outcome data, claims for reduced harm status of PREPs depend heavily on standard machine yield smoke constituent data. Two prototypes of the new carbon-filtered PREP Marlboro UltraSmooth (MUS) were investigated using both standard (FTC/ISO) and intensive (Health Canada) machine methods to measure gas/vapor- and particulate-phase smoke constituents. Basic physical design characteristics that may influence smoke constituent yields, such as ventilation, pressure drop (resistance to draw), quantity of tobacco, and quantity and type of carbon, were measured. The possible presence of added chemical flavorant compounds was investigated using gas chromatography-mass spectroscopy. MUS prototypes were found to have several key differences in physical design compared with a conventional cigarette, including higher ventilation, lower draw resistance, and in the case of the Salt Lake City prototype, the use of vitreous carbon beads and the presence of chemical flavorants on both the beads and an embedded filter fiber. When tested under the standard regimen, gas-phase constituents of MUS prototypes were reduced compared with a conventional low-yield cigarette. However, far smaller reductions in gas-phase constituents were observed under the intensive regimen, suggesting that the carbon technology used in MUS is less effective when smoked under more intensive conditions. Particulate-phase constituents were not reduced by the carbon filter under either machine-smoking regimen. The data suggest that MUS has been designed to reduce toxic yields while preserving consumer appeal. However, MUS is less effective in reducing toxic smoke constituents when smoked under intensive conditions.