While the apparent optical properties of sea ice vary with ice type and temperature throughout the annual cycle, they depend more fundamentally on how inclusions of brine, gas, precipitated salts, and other impurities are distributed within the ice. Since little is known about these distributions or about how they evolve with temperature, experiments were designed to collect detailed information on the microstructure of Arctic sea ice over a wide range of temperatures. An imaging system, capable of resolving inclusion sizes of less than 0.01 mm in diameter, was used to examine the microstructure of first-year ice in a temperature-controlled laboratory. Experiments were initially carried out at -15¿C to obtain size distributions for brine inclusions and gas bubbles in cold ice. Brine inclusion dimensions were found to range from less than 0.01 mm to nearly 10 mm, with number densities averaging about 24 pockets per mm3. This is an order of magnitude larger than number densities previously reported. Gas bubbles in the samples were generally smaller than 0.2 mm and had number densities of approximately 1 per mm3, also an order of magnitude larger than previously reported. Large changes in microstructure were observed as samples were cooled to -30¿C and subsequently warmed to -2¿C. Observational results document the thermal evolution of the ice, as well as interactions between brine inclusions, gas bubbles, and precipitated salts. The link between the structural and optical properties of sea ice is closely tied to the total cross-sectional area of the inclusions. We show that this quantity increases dramatically when the ice cools below -23¿C or warms above -5¿C, but because changes in brine inclusions offset changes in precipitated salts, it remains surprisingly constant between these temperatures.