The effect of the single, site-specific interstrand cross-link formed by cisplatin or transplatin on the thermal stability and energetics of 20-base pairs DNA duplex is reported. The cross-linked or unplatinated 20-bp duplexes were investigated with the aid of differential scanning calorimetry, temperature-dependent UV absorption and circular dichroism. The cross-link of both platinum isomers increases the thermal stability of the modified duplexes by changing the molecularity of denaturation. The structural perturbation due to the interstrand cross-link of cisplatin increases entropy of the duplex and in this way entropically stabilizes the duplex. This entropic cross-link induced stabilization of the duplex is partially, but not completely compensated by the enthalpic destabilization of the duplex. The net result of these enthalpic and entropic effects is that the structural perturbation due to formation of the interstrand cross-link by cisplatin induces a decrease in duplex thermodynamic stability with this destabilization being enthalpic in origin. By contrast, the interstrand cross-link of transplatin is enthalpically almost neutral with the cross-link-induced destabilization being entirely entropic in origin. These differences are consistent with distinct conformational distortions induced by the interstrand cross-links of the two isomers. Importantly, for the duplex cross-linked by cisplatin relative to that cross-linked by transplatin, the compensating enthalpic and entropic effects almost completely offset the difference in cross-link-induced energetic destabilization. It has been proposed that the results of the present work further support the view that the impact of interstrand cross-links of cisplatin and transplatin on DNA is different which might be also associated with distinctly different antitumor effects of these platinum compounds.