Ultrafine particles (diameter less than 100 nm) are primary suspects for enhanced negative health effects on humans. Measuring the chemical composition and physical properties of ultrafine particles on-line, continuously, and accurately is particularly challenging because of their typically low mass concentration (PM_0.1) and susceptibility to interference from larger particles. The few past PM_0.1 chemical composition measurement studies have used cascade impactors and at least daily temporal resolution. In this study we perform for the first time high temporal measurements of the composition and sources of PM_0.1 using an aerodynamic aerosol classifier (AAC) to separate PM_0.1 from larger particles. A high-resolution time of flight aerosol mass spectrometer (HR-ToF-AMS, for sulfate, nitrate, organics, chloride), a single particle soot photometer (SP2-XR, for black carbon) and an Xact625i (for elements) are also used.</p> <p>Ambient PM_0.1 composition measurements were conducted in a suburban area in Greece to test the system. The hourly PM_0.1 levels varied from 0.4 to 1.5 &mu;g m^-3, with an average of 0.7 &mu;g m^-3. Most of the PM_0.1 (45 %) was organic aerosol (OA). On average, sulfates contributed 14 %, ammonium 7 %, nitrate 3 %, and black carbon 4 % to PM_0.1. Calcium (Ca) showed a surprising high average contribution to PM_0.1 (18 %). The rest of the detected elements were Fe, K, Zn and Ti, contributing together 7 %. Source apportionment analysis showed that most of the PM_0.1 OA, during this summertime period, was oxygenated OA (90 %), with 70 % being less oxidized and 20 % being more oxidized, while only 10 % was fresh hydrocarbon-like OA.