The changes in the minority carrier lifetime for heat-treated silicon crystals at relatively low temperatures (200-600 degrees C) were observed. The minority carrier life-time of n-type floating-zoned swirl-free and swirl-containing, n-type Czochralski-grown swirl-containing and p-type floating-zoned swirl-containing, silicon crystals abruptly decreased between 400 and 500 degrees C, and the activation energy for defect formation in this temperature range was about 1.3 eV in these crystals. However, in p-type floating-zoned swirl-free silicon crystals, the behaviour of the minority carrier lifetime was different; there was an increase in the lifetime centred around 350 degrees C and an abrupt decrease above 600 degrees C. The defect formation energies were 1.39 eV between 300 and 350 degrees C, 3.30 eV between 350 and 400 degrees C, and 3.25 eV between 580 and 620 degrees C. The influences of swirl defects on the changes in the minority carrier lifetime during the heat treatments are also described.