​​​The physical state of the catalyst and its impact on the growth of single-walled carbon nanotubes (SWNTs) is the subject of a long-standing debate. We addressed it here using in situRaman spectroscopy to measure Fe and Ni catalyst lifetimes during the growth of individual SWNTs across a wide range of temperatures (500–1400 °C). The temperature dependence of the Fe catalyst lifetimes underwent a sharp increase around 1100 °C due to a solid-to-liquid phase transition. By comparing experimental results with the metal–carbon phase diagrams, we prove that SWNTs can grow from solid and liquid phase-catalysts, depending on the temperature.