The process parameters of an inductively coupled thermal plasma used for nanopowder synthesis are experimentally investigated using various plasma diagnostics and in situ powder monitoring methods. An enthalpy probe technique is applied to characterize the plasma properties under particle-free conditions. The nanoparticle synthesis from microscale alumina precursors is monitored in situ by optical emission spectroscopy and laser light extinction measurements to investigate the powder evaporation. The synthesized powders are collected in a sampling unit and characterized ex situ by particle size analysis as well as by electron microscopy. At low flow rates of the torch central gas, higher plasma enthalpy, a laminar powder flow and increased evaporation of the precursor have been observed. A precursor- and an energy-deficient regime related to the precursor feed rate and plasma enthalpy are found from the emission line intensities of aluminium metal vapour. The number fraction of plasma-treated precursors, which is an important process parameter, is calculated from the precursor number density obtained from laser extinction measurements.