High‐resolution, room‐temperature, infrared absorption measurements of boron‐ and lithium‐compensated silicon are reported. Isotopic shifts of the observed absorption bands with changes in lithium and boron isotopes are given. The absorption strengths of the bands at 681, 655, 584, 564, and 522 cm−1associated with lithium‐boron pairs, are all proportional to the impurity concentration from ∼2×1018to 1020cm−3. Weak bands associated with unpaired boron are also observed in the same samples at 644 and 620 cm−1. The 522 cm−1band shifts to 534 cm−1with change in lithium isotope and is nearly independent of the boron isotope. The frequencies of the remaining bands depend almost completely on the boron isotope.