Determination of the interstitial oxygen concentration in silicon is one. of important elements of its characterization. In the room temperature interstitial oxygen has an absorption level for the —1 wavelength X=9.05 im Cv = 1106 cm ). This level is very well seen because defectless silicon has low absorption in this range. Thus, mainly used method for interstitial oxygen concentration determination is based on dependence between the absorption coefficient of the oxygen peak as measured and interstitial oxygen concentration N [1, 2, 3]. In measurements of N some difficulties associated with absorption on phonons appear. At low silicon resistivities additional difficulties connected with the absorption on free carriers appear [4, 5] and the measurements at very low temperatures are necessary [6, 7]. For the absorption measurements an external source of infrared radiation (IR) of high stability (e.g. CO2 - laser) and thick samples with very well polished both surfaces  are required, which is often impossible in the case of commercial wafers. In this work a new optical method enabling determination of the interstitial oxygen concentration N, as well as its spatial distribution in silicon wafers is described. This method is based on the thermal radiation (TR) measurement of the investigated material in the range of the interstitial oxygen absorption [81. According to the Kirchhoff's law the TR emission depends on the absorption properties of the material which enables in certain conditions to estimate the oxygen concentration. For this method an external source of IR is not necessary, and measurements can be carried out on wafers with metallized or rough back surfaces. Creating conditions enabling the measurement of TR radiation of the wafer is realized by heating or cooling a screen placed behind the wafer. With this method one can measure temperature dependence of interstitial oxygen concentration. The measurements can be carried outwith the use of the spectrophotometer, a scanning IR microscope or a thermovision camera with suitable adjusted spectral range.