[en] The fuel cladding material for isotope heat sources operating above 1300⁰C is currently an Ir--0.3 percent W alloy. The ductility of this material under high-temperature impact accident conditions is reduced by large grain sizes. Grain size as a function of time and temperature is required to determine the effects of high-temperature exposure during fabrication, assembly, ground testing, and operation of the heat source. The average grain diameter and the number of grains across the 0.64 mm (0.025 in.) cladding-wall thickness of several batches of Ir--0.3 percent W sheet is reported as a function of time at 1583 K (1310⁰C), 1688 K (1415⁰C), and 1808 K (1535⁰C) up to 2000 hr. Growth rates were measured both perpendicular and parallel to the sheet surface in longitudinal cross sections of sheet samples. In alloys with Th, Al, Fe, Ni, and Rh dopant additions, the grain structure was elongated and grain growth was retarded. Grain size was measured by the line intercept method and the data were described by the grain growth equation, d²--d₀² = kt. An activation energy for grain growth was determined for each batch, in both the perpendicular and parallel directions. Abnormal grain growth was not observed within the time and temperature ranges of this experiment