A floating catalyst chemical vapour deposition method for producing nanotubes, the method including: supplying a nanotube-material precursor and a catalyst precursor, heating said precursors and injecting said precursors into a heated reaction chamber containing a process gas; pyrolysing the catalyst precursor within the reaction chamber to produce catalyst particles; and pyrolysing the nanotube-material precursor within the reaction chamber in the presence of the catalyst particles in order to produce nanotubes; wherein the method further comprises controlling the size of the catalyst particles at the point of pyrolysis of the nanotube-material precursor by controlling the operational parameters of the reaction chamber and/or of the precursor supplies. A corresponding system for producing nanotubes is also provided. Further provided is an injector subsystem for attachment to a reaction chamber, the injector subsystem comprising: a first portion having a first set of injection pipes for receiving species for delivery into the reaction chamber; and an interface portion that is removably attachable between the first portion and the reaction chamber in use, the interface portion comprising at least one injection pipe arranged to receive the species provided by the first set of injection pipes and to inject said species into the reaction chamber in use;the injector subsystem being operable in use such that, with the interface portion removed, said first set of injection pipes are operable to individually inject their respective species into the reaction chamber, and with the interface portion attached, a pre-mixing chamber is defined by the interface portion in combination with the first portion, for pre-mixing the species provided by the first set of injection pipes prior to the species entering the at least one injection pipe of the interface portion.