From nano‐test tube chemistry we determine details of low temperature growth of carbon nanotubes. Single‐walled carbon nanotubes of various diameters are grown at different temperatures from a Pt organometallic precursor encapsulated inside template single‐walled tubes. At the low temperature limits where the reactions are slower and milder, the inner‐tube diameters are optimally determined by the outer‐tube diameter. The growth temperature is determined for a given tube chirality and is found to be dependent on the diameter. It is lower for thinner tubes and can be as low as 600 °C for the (7.3) tube. In situ nano‐test tube chemistry within a state‐of‐the‐art microscope provides an idealistic nano‐clean room experiment, which allows monitoring of a snapshot in the catalytic growth of a single‐walled tube out of a Pt nanocrystal.