Concentrations of formaldehyde and other volatile organic compounds (VOCs) were monitored over a 2-year period in the main rooms of four unoccupied test houses and in the outdoor air. During the construction and furnishing of the buildings 30 samples of materials were collected and subsequently tested using environmental test chambers to determine amounts of form aldehyde and other VOCs emitted. Concentrations of VOCs in the building were initially high and declined quickly during the first 6 months after construction. Formaldehyde concentrations showed a slower rate of decline with strong seasonal effects resulting in highest concentrations during summer months. The most important source of benzene and toluene was the outdoor air whereas almost all materials were potential sources of the wide range of VOCs found in the indoor air. Some materials such as paints and flooring are identified as the most important sources of the major VOCs occurring in the indoor air.