In a method suitable for tissue fusion or coagulation, after the start of treatment of the tissue, i.e., after completion of a first phase, a second phase is commenced, during which the biological tissue is treated for a certain specified process time with moderate energy input. The specification of a negative internal resistance of a supplying source enables the process in the second phase to have a treatment time that remains constant and thus avoids premature drying-out of the tissue. A sufficient and reliable bonding of the participating proteins in the moist milieu is reached.