This letter reports on a physical method to produce highly pure, size-controlled and well-dispersed spherical gold nanoparticles (NPs) in Therminol VP-1 by pulsed laser ablation in liquids (PLAL) using a 30 fs Ti: Sapphire laser at a fluence of 1 J cm− 2. A second photo-fragmentation of the ablated colloid solution by subsequent treatment with the same laser light yields a mean size and size dispersion of the NPs of 58±31 nm. A study of the nanofluid properties reveals a low agglomeration over time and an enhancement of thermal conductivity of the base fluid by up to 4%. These results improve the characteristics of current nanofluids in thermal oils that may have a potential impact on the improvement of the efficiency of harvesting of solar light.