This investigation presents measurements of isothermal ssDNA oligomer hybridization enthalpy at various DNA length, GC contents, temperatures, and salt concentrations by isothermal titration calorimetry (ITC) at temperatures below the melting temperature of the DNA. The study aimed to reveal the hybridization mechanism of ssDNA for gene chip applications. The role of hydrogen bonds between complementary bases, the stacking forces between bases, the electrostatic repulsion between strands, the dehydration and conformational rearrangement of ssDNA in hybridization are estimated from the interaction enthalpy. (c) 2005 Elsevier B.V. All rights reserved.