We studied the mechanisms of spontaneous termination of atrial fibrillation in a biophysical model of human atria, during the eight seconds preceding termination. The earliest detectable changes in the cycle length and the number of wavefronts occurred about three seconds prior to termination. We compared the mechanisms involved in the right and left atrium and investigated the effects of atrial geometry on the termination processes. We observed that cycle length started to increase 800 ms earlier in the left atrium than in the right atrium. Similarly, the number of wavefronts decreased even 1800 ms earlier in the left atrium than in the right one. Significantly fewer episodes terminated in the left atrium. Four areas of the atrial geometry showing distinct termination mechanisms were identified.