The comparative experiments on conduction/convection in a ferro-nanofluid (Prandtl number 700) and its organic base, transformer oil (Prandtl number 300), are performed in a spherical cavity heated from above/below. The experiments show that the conductivity of a ferro-nanofluid increases compared to that of its base. However convection and the associated heat transfer are suppressed in a ferro-nanofluid relative to those in transformer oil. The investigation also reveals the existence of a hysteresis and oscillatory flow regimes near the convection onset in ferro-nanofluids and commercial transformer oil that fundamentally differ their behaviour from that of ordinary monofluids. (C) 2016 Elsevier Ltd. All rights reserved.