As part of the Pollution des Vallées Alpines (PoVA) program, the number concentration and size distribution of the total PM2.5 population, as well as the subset that is water-insoluble, were measured in the Chamonix and Maurienne Valleys of the French Alps. This program included both summer and winter intensive campaigns in January and June-July 2003. The water-insoluble aerosol (WIA) measurements were performed using a new real-time technique developed at the Georgia Institute of Technology. The concentration of both the total aerosol population and WIA was found to be highly variable on timescales as brief as a few minutes and was generally much higher in winter than in summer. In addition, the fraction (by number) of aerosols which are not soluble in water was also found to be variable and slightly higher in winter. The average insoluble fraction in the size range of 0.25-2.0 pm was approximately 11% in Chamonix in both winter and summer. In the Maurienne Valley the insoluble fraction was 10% in winter and 8% in summer. Although the number concentration of WIA is dominated by particles smaller than 0.35 μm, a peak is consistently observed between 0.4 and 0.5 μm. Size-resolved filter samples were also collected as part of the PoVA program and were analyzed for EC/OC mass. Comparison of these two data sets suggests that WIA in these valleys is dominated by elemental carbon emissions from motor vehicle traffic, although mineral aerosols also contribute during the summer season. Further analysis also suggests that water-soluble and water-insoluble aerosols have sources which are independent of one another both temporally and spatially. Copyright 2006 by the American Geophysical Union.