The U.S. Consumer Product Safety Commission (CPSC) is concerned about the hazard of acute residential carbon monoxide (CO) exposures from portable gasoline powered generators that can result in death or serious and/or lasting adverse health effects in exposed individuals. As an initial approach to characterizing these hazards, CPSC measured the emissions from generators by testing them in a small test chamber (Brown 2006). CPSC subsequently contracted with the University of Alabama (UA) to develop and construct low CO-emission prototype generators using off-the-shelf technologies installed on commercially-available portable generators. Under an interagency agreement with CPSC, NIST conducted a series of tests to characterize the indoor time course profiles of CO concentrations resulting from portable generators operating in the attached garage of a home under various use and environmental conditions, to evaluate the performance of low CO-emission prototype generators, and to provide model validation data. The data was also used as input to a simulation analysis conducted to examine the potential performance of the low CO-emission prototypes under a wider range of operating conditions.