Beam splitters for 10-mum lasers are discussed and then applied to the precision measurement of high average powers. In particular, beam splitter stability has been investigated in various materials over the 20-600-W power range with power densities up to 1 kW/cm(2). The absolute beam splitter ratios are given along with the achieved measurement precisions. The semiconductors investigated were GaAs, CdTe, and ZnSe in addition to one alkali-halide KCl. Standard deviations for the beam splitter ratios of 1% over the power range were typical. Absolute ratios agree with the predictions from Fresnel's equations to 1% or better. The best measurement was made on ZnSe when a standard deviation of 0.4% was obtained for the measurement of a ratio that agreed with a calculation from Fresnel's equations to better than 0.5%.