Multiple transmission-reflection (MTR) infrared (IR) spectroscopy was established as a new IR method to measure the interstitial oxygen and substitutional carbon contents of polycrystalline silicon wafers at ambient temperature. The superiority of MTR-IR to conventional IR was verified through practical measurements of polycrystalline silicon samples. With single incidence, the following characteristics of the former were superior to those of the latter. (1) The areas of the absorption peaks of interstitial oxygen at 1107 cm(-1) and substitutional carbon at 605 cm(-1) can be amplified linearly with transmission times N between 10 and 12, thereby extending the limit of detection of oxygen and carbon to approximately one order of magnitude lower. (2) The strength of the interference fringes of the baseline can be decreased to 5 times that of the single incidence for a 1 mm polycrystalline silicon slice. (3) In contrast to conventional IR method in which data are gathered from one sampling point at each measurement, MTR-IR collects information from many sampling points in a long sample for one-time measurement. In conclusion, MTR-IR is more sensitive, reliable, and reproducible than the conventional IR method in terms of measuring the interstitial oxygen and substitutional carbon contents of polycrystalline silicon.