The capillary flow hydrothermal reactor system for the UV-vis spectrophotometric detection system (CHUS) has been applied to inspect the behavior of DNA at temperatures up to 300 degrees C. Double-stranded DNA (dsDNA) was monitored with and without ethidium bromide in the presence of MgCl2. The ratio (A(+DNA)/A(-DNA)) of the absorbance of EB with DNA (A(+DNA)) to that without DNA (A(-DNA)) increased steeply over I at temperatures around 100 degrees C, which was not observed in the absence of MgCl2. This was found to be due to the decrease of solubility of DNA in hot water, where dsDNA is converted to single-stranded DNA and become insoluble. At temperatures over 175 degrees C, DNA becomes soluble again because of the degradation of long insoluble DNA polymers to short soluble DNA oligomers. This study points out the importance of solubility of DNA for life at extremely high temperatures as well as the stability of DNA. (C) 2007 Elsevier B.V. All rights reserved.