Two infrared absorption bands attributed to substitutional boron‐phosphorus pairs in silicon are observed. The bands are close to the single, isolated boron band and all show approximately the same frequency shift with change in boron isotope. The pair bands occur near 599.7 and 629 cm−1for11B and 622.9 and ∼655 cm−1for10B. The results are compared with the theory of Elliott and Pfeuty. The number of pair bands, their isotope shift, and their proximity to the isolated B band are in agreement with theory. The &Dgr;&ngr; ∼ 30 cm−1is an order of magnitude larger than predicted by the isotopic model indicating changes in force constants.