Weakly-ferromagnetic hematite is a cheap, environmentally friendly and thermodynamically stable iron oxide, and 1D hematite nanorods (NRs) have been studied for a wide range of applications because their magnetic properties are greatly dependent on NR size and shape. Herein, the hydrothermal synthesis (HS) of hematite NRs is investigated using a combination of analytical techniques. Development of a novel, valve-assisted, hydrothermal pressure vessel, which allows for the rapid quenching of hydrothermal products as a function of reaction time and known reaction temperature, provided fundamental insight into the anisotropic crystal growth mechanism of the acicular hematite NRs. The hematite NR growth mechanism was found to be a two stage process: 1) the growth and dissolution of intermediate ß-FeOOH NRs, alongside precipitation of primary hematite nanoparticles (NPs), and 2) the agglomeration and coarsening of primary hematite NPs into hematite NRs. The investigation of rapidly quenched ß-FeOOH and hematite HS reaction products, heat treated in situ within a transmission electron microscope, provides direct evidence for the hydrothermal growth mechanism of lenticular hematite NRs.