Simultaneous indoor and outdoor measurements of aldehydes were made at 6 residential houses located in a suburban New Jersey area during the summer of 1992. Each house was measured for six days and controlled for ventilation and gas combustion conditions during the study. Formaldehyde, acetaldehyde, and seven other aldehydes were identified in the residential air. The study presents the first measurements of nine aldehyde species in both indoor and outdoor air. The total concentrations of the nine aldehydes were 19.12 ± 10.88 ppb outdoors and 62.57 ± 21.75 ppb indoors. Formaldehyde was the most abundant aldehyde. Except for propionaldehyde, the indoor concentrations were found higher than the outdoor concentrations for all the other compounds, indicating the presence of significant indoor sources such as direct emissions and indoor chemical formation. Ozone concentrations were measured simultaneously during the study, and it was observed that several aldehydes can be generated through indoor ozone chemistry. The study provided evidence to support laboratory results obtained by other investigators that predicted aldehyde generation through indoor ozone chemistry. The residential exposures to formaldehyde and total aldehydes were assessed based upon some assumptions, and the outdoor exposures were negligible compared to the indoor exposures. © 1994, American Chemical Society. All rights reserved.