Polyelectrolyte complexes of calf-thymus DNA and polycations, such as PDADMAC, IONENE, and P4VP were formed and analysed with respect to their thermodynamical stability. The UV-VIS spectrographic melting curves yield two denaturation temperatures, T-m,T-1 and T-m,T-2. This is explained as follows: DNA molecules contain two kinds of repeat unit sequences. There are sequences that are not complexed, they denature at T-m,T-1; the other DNA repeat units are complexed with polycation repeat units, they denature at T-m,T-2 Thereby, T-m,T-2 is much larger than T-m,T-1. That is, polycation binding stabilises calf-thymus DNA. Surprisingly, the absolute value of T-m,T-1 depends neither on the degree of complexation nor on the polycation type. T-m,T-1 agrees quite well with the denaturation temperature of the pure, uncomplexed DNA. Also T-m,T-2 does depend not on the degree of polycation complexation, but on the polycation type. One observes T-m,T-2(PDADMAC)> T-m,T-2(P4VP) > T-m,T-2(IONENE). The probable reason for this series is the polycation molar mass. T-m,T-2 increases slightly as M-w is increased. While T-m,T-1 depends on the concentration of the added NaCl-salt, T-m,T-2 does not. This is explained using the fact that the complexed DNA repeat units are electrically neutral, so that there are no salt/charge interactions. Measurements in aqueous/organic solvent mixtures show that both T-m,T-1 as well T-m,T-2 decrease continually as the content of the organic solvent is increased. Very marked is this effect for water/N-methylformamide, Al w(NMF) approximate to 40%, T-m,T-1 and T-m,T-2 are nearly half as large as in pure water.