Aerosol mass size distributions of elemental carbon (EC) and organic carbon (OC) were measured to determine the extent to which carbon speciations within ambient aerosols differ between urban and over-water atmospheres. Samples were collected during twenty-one 12-h periods in urban Chicago and over Lake Michigan during the July 1994 and January 1995 sampling intensives. Total particle bound concentrations (sum of all size fractions) of elemental and organic carbon ranged from 0.15 to 0.96 and from 0.94 to 3.04 g m −3, respectively. On average, organic material (OM=1.4×OC) accounts for 18% of the total aerosol mass collected, but ranges from 10% (over-water, winter) to 39% (urban, summer). With regard to individual size fractions, organic matter ranges from 3% (>12 μm over-water, winter) to 49% (0.15–0.45 μm, urban, summer) of the particulate matter mass in the size class. Geometric mean aerodynamic equivalent diameters (GMDs) range from 0.72 to 2.4 μm for suspended particulate matter ( Σ-PM), from 0.52 to 1.4 μm for EC, and from 0.60 to 1.9 μm for OC. Elemental and organic carbon GMDs are larger in the urban atmosphere than over the water during winter, while this trend is not observed in Σ-PM. Furthermore, geometric standard deviations are larger at the urban location in January for both EC and OC, indicating broader size distributions of both species under winter conditions due to greater quantities of EC and OC in large particles.