The synthesis and in-situ organic surface modification of spinel cobalt aluminate nanoparticles were carried out under subcritical reducing hydrothermal conditions (250 degrees C, 30 MPa). Single nanoparticles of cobalt aluminate were obtained using a continuous flow-type reactor; in contrast, Co1-xAlx layered double hydroxide (Co-Al-LDHs) became the main product when a batch-type reactor was used. The addition of a surface modifier, hexanoic acid, to the flow-type reaction system changed the hydrophilic surface of the nanoparticles to a hydrophobic surface. Cobalt aluminate nanoparticles modified with hexanoic acid were well dispersed in hexane. The in-situ surface modification during the synthesis of the nanoparticles reduced the particle size from 15 nm to 10 nm. (C) 2012 Elsevier B. V. All rights reserved.