Using a fluorescence-based method, we have determined the number of thiol-derivatized single-stranded oligonucleotides bound to gold nanoparticles and their extent of hybridization with complementary oligonucleotides in solution. Oligonucleotide surface coverages of hexanethiol 12-mer oligonucleotides on gold nanoparticles (34 +/- 1 pmol/cm2) were significantly higher than on planar gold thin films (18 +/- 3 pmol/cm2), while the percentage of hybridizable strands on the gold nanoparticles (1.3 +/- 0.3 pmol/cm2, 4%) was lower than for gold thin films (6 +/- 2 pmol/cm2, 33%). A gradual increase in electrolyte concentration over the course of oligonucleotide deposition significantly increases surface coverage and consequently particle stability. In addition, oligonucleotide spacer sequences improve the hybridization efficiency of oligonucleotide-modified nanoparticles from approximately 4 to 44%. The surface coverage of recognition strands can be tailored using coadsorbed diluent oligonucleotides. This provides a means of indirectly controlling the average number of hybridized strands per nanoparticle. The work presented here has important implications with regard to understanding interactions between modified oligonucleotides and metal nanoparticles, as well as optimizing the sensitivity of gold nanoparticle-based oligonucleotide detection methods.