Structurally, sea ice consists of an ice matrix with inclusions of brine and air. A quantitative description of the size of these inclusions is critical for both interpreting and modeling the electromagnetic properties of sea ice. Photomicrographs of ice thin sections were analyzed using a personal computer-based image-processing system to determine the number of inclusions, the inclusion size distributions, and statistics for brine pockets in young ice and first-year ice and for air bubbles in a multiyear hummock. Inclusions ranging in size from thousandths of square millimeters to a few square millimeters were measured. In all cases a two-parameter lognormal distribution fits the cumulative inclusion size distributions well (correlation coefficient greater than 0.99). This includes brine pockets in both granular and columnar ice and a range of brine volumes from 2% to 40%. As ice warms, and its brine volume increases, the size distribution shifts toward larger brine pockets. This increase in brine pocket size is particularly pronounced for brine volumes greater than 10% as individual brine pockets coalesce. Air bubbles are much larger than brine pockets, with mean major axis lengths of the order of millimeters for air bubbles and tenths of a millimeter for brine pockets. Observations of inclusion shape factors indicate that, in general, brine pockets are more elongated than air bubbles. ¿ American Geophysical Union 1996