The interaction of Li atoms with O impurities and with defects produced in Si by 1.5 MeV and 47 MeV electrons was studied using infrared spectroscopy. Li was diffused in low oxygen‐containing uncompensated Si with initial resistivity in the range 15–28 500 &OHgr;·cm and in high oxygen‐containing Si with initial resistivity of 15–200 &OHgr;·cm. The Li concentration in the samples varied from about 1016to ≲1019Li atoms/cm3. New radiation‐induced bands at 1.4 and 1.7 &mgr; are observed. Divacancy production as measured by the 1.8 and 3.3 &mgr; defect bands is markedly decreased in samples of high Li content with concomitant growth of the Li‐associated 1.4 and 1.7 &mgr; bands. Annealing of the 1.4 and 1.7 &mgr; bands strongly depends upon the Li concentration. Recovery occurs at a temperature of 150°C for the 1.4 &mgr; band, and at 350°C for the 1.7 &mgr; band, for samples containing ∼6×1017Li/cm3. In samples containing ∼2×1016Li/cm3, the bands at 1.4 and 1.7 &mgr; disappear after annealing at 330° and 470°C, respectively. Sharp Li‐ and O‐associated defect bands in the 700–1000 cm−1range are produced only in high O‐containing samples, and the presence of Li has a pronounced effect on the production and recovery of theA‐center defect (830 cm−1band). In samples of high Li content, the predominant absorption is due to free carriers and is found to be in agreement with predictions of theory. A search for defect absorption bands attributable to vibrational modes of the isolated interstitial Li+ions yielded negative results.