Infrared absorption in GaAs∕AlxGa1−xAs multiple quantum wells is investigated using a polarizer. Two main peaks, with wave numbers 723 and 1092cm−1, are observed. The peak with wave number 1092cm−1 corresponds to the 0→1 intersubband transition, while the other one corresponds to the intrasubband transition. The polarized absorbance is one order of magnitude higher than the unpolarized one. The authors attribute the intrasubband transition to the plasma oscillation in the quantum wells.