The carrier lifetime is very sensitive to electrically active defects. Since the recombination activity of a defect is determined by its energy level and the product of capture cross section times defect concentration, the carrier recombination lifetime can be significantly affected by the presence of electrically active defects, even if the defect concentration is below the detection limit of deep level transient spectroscopy (DLTS): The metastable defect in boron-doped Cz-silicon is a prominent example for such a highly recombination active defect. A versatile method to detect directly the presence and the nature of recombination-active defects are temperature- or injection-dependent lifetime measurements. In the present work the applicability of lifetime measurements as a spectroscopic method is examined. As temperature-dependent lifetime measurements allow a direct determination of the energy level, we applied this tool on intentionally metal-contaminated samples using the microwave-detected photoconductance decay method (MW-PCD). The quality of the lifetime measurements as a spectroscopic method is demonstrated. Applying the temperature-dependent lifetime spectroscopy (TDLS) to boron-doped Cz-silicon, we determined for the metastable defect in its passive state an energy level of 0.069 e V.