Hydrophobic silver and titanium (IV) oxide nanoparticles (commercial Ag and TiO2 NPs with average particle sizes of 17 and 19nm, respectively) were quantitatively transferred into organic phase in natural water samples. Five NP surface modification and solvent extraction agents (reagents) types, mercaptocarboxylic acid, alkylamine, mediator solvent, extraction solvent, and surfactant, were investigated and optimized with three-level orthogonal array design (OAD), an OA27 (3(13)) matrix. The most favorable reagents and experimental conditions were then examined. The best extraction efficiencies of 78.6 and 73.7% were obtained for 1mgL(-1) citrate-stabilized Ag and TiO2 NPs, respectively, with 0.5mM of 11-mercaptoundecanoic acid, 1.5mM of octadecylamine, 1mL of methanol, 150μL of cyclohexane, 0.05mM of tetra-n-octylammonium bromide, pH=8.0, adsorption time of 2h, sonication time of 3min, and centrifugation time of 10min. Enrichment factors were 97 and 83, for Ag and TiO2 NPs, respectively. The optimum extraction conditions were successfully applied to genuine water samples at spiking levels of 2-100μgL(-1) of Ag and TiO2 NPs. The relative recoveries of (69.0-85.1)% and (61.5-78.5)% were obtained for Ag and TiO2 NPs, respectively. The extracted surface-modified NPs were characterized with transmission electron microscopy, selected area electron diffraction, energy-dispersive X-ray, ultraviolet-visible, and Fourier transform infrared spectroscopic techniques. Based on the results, efficient ligand exchange and acid-base pair formation were observed on the NP surface without significant change in its original properties. The organic phase was microwave digested, and analyzed with inductively coupled plasma (ICP) optical emission spectroscopy and ICP mass spectrometry (ICP-MS). Detection limits of ICP-MS analyses of Ag and TiO2 NPs were 0.02 and 0.07μgL(-1), respectively.