This study was undertaken to assess indoor air quality in two similar, new houses being evaluated for energy performance. One house (A) was built conventionally. The other (B) was an energy-eff icient structure. Air samples for individual volatile organic compounds (VOCs), total VOCs (TVOC) and formaldehyde were collected following completion of the interiors of the houses and on several occasions during the following year. Ventilation rates were also determined so that source strengths of airborne contaminants could be estimated with a massbalance model. There were no substantial differences in indoor air quality between the houses. The N O C concentrations in House A ranged from 1,700 - 4,400 pg m-3, with the highest value coinciding with the lowest ventilation rate. The TVOC concentrations in House B were 2,400 - 2,800 pg m-3. These values are elevated compared to a median value of 700 pg m-3 measured for a large residential study. Formaldehyde concentrations ranged up to 74 pg 177-3. The dominant VOC in both houses was hexanal, an odorous chemical irritant. The concentrations of acetone, pentanal, toluene, alpha-pinene and other aldehydes were also relatively high in both houses. The source strengths of many of the analytes did not decline substantially over the course of the study. For example, the initial TVOC source strength for House A was 385 mg h-1. Eight months later it was still 230 mg h-1. Interestingly, the source strengths of pentanal and hexanal increased over this period in House A. In House B, the source strengths of all VOCs declined over a one-year period. However, for a number of compounds, the decline was less than 50 percent. A chamber study of oriented strand board (OSB) collected from House B found that this material emitted formaldehyde, acetone, hexanal and a few other VOCs also detected in the houses. Based on the chamber data, the OSB was estimated to contribute substantially to concentrations of formaldehyde and acetone in the houses. The results also suggested that OSB was not the dominant source of pentanal, hexanal and alpha-pinene, all of which had elevated emissions in the houses, possibly from a single source.