To investigate the potential soiling hazard to works of art posed by the deposition of airborne particles, time-resolved measurements were made of the size distribution and chemical composition of particles inside and outside of three southern California museums. The measured indoor aerosol characteristics agree well with predictions of a mathematical model of indoor aerosol dynamics based on measured outdoor aerosol characteristics and building parameters. At all three sites, the fraction of particles entering from outdoor air that deposit onto surfaces varies strongly with particle size, ranging from a minimum of 0.1-0.5per thousand for particles having a diameter in the vicinity of 0.15 mum to greater than 90per thousand for particle larger than 20 mum in diameter. Deposition calculations indicate that, at the rates determined for the study days, enough elemental carbon (soot) would accumulate on vertical surfaces in the museums to yield perceptible soiling in as little as 1 year at one site to as long as 10-40 years at the other two sites.