In somatic gene therapy, new concepts for the transfer of DNA into specific cell nuclei are of interest. Inorganic nanoparticles have an interesting potential as DNA carrier system due to the possibility to tailor their surface reactivity and the electrical surface potential (zeta potential) that can be obtained by the surface modification. SiO₂ nanoparticles have been chosen for reasons of low toxicity. In order to obtain positively charged nanoparticles, basic surface groupings have been selected. Different types of alkoxysilanes and amines have been tested for surface modification of SiO₂ nanoparticles. The zeta potential at pH=7.4 could be varied from −38.8 mV (unmodified 10 nm particles) to +20 mV (gamma-aminopropyl-triethoxysilane) and to +49.8 mV (glycidoxypropyltrimethoxysilane and ethylendiamine). DNA could be completely immobilized at the nanoparticle surface and nanoparticle/DNA ratios between 2 and 15 (w/w) were obtained. The interaction of DNA with the particle surface correlates with an increasing number of modifier molecules on the particle surface.