The electronic properties of two dominant sulfur‐related deep donor levels in silicon have been investigated using capacitance and dark current transient techniques. One of the centers, theBcenter, has an enthalpy &Dgr;Hnof 0.32 eV and an electron‐capture cross section &sgr;tnBof 2×10−15cm2at 100 K. &sgr;tnBvaries with temperature asT−3.3. A plot of log etnAvs 1/Tfor the other center, theAcenter, shows a ’’thermal activation energy’’ of 0.59 eV. &sgr;tnAis estimated to be larger than 10−14cm2. The nature of theAandBcenters is discussed.