The pyrolysis of polycarbosilane (PCS), a ceramic precursor polymer, at temperatures up to 700 °C under an inert atmosphere results in the development of amorphous microporous materials which have a number of potential applications, such as gas separation membranes. This paper investigates the development of microporosity during pyrolysis under nitrogen, at temperatures ranging from 300 to 700 °C, of both the cross-linked and non-cross-linked starting materials. The products are characterised by nitrogen adsorption, to determine surface areas and pore volumes, solid-state NMR, electron microscopy and FTIR, and their formation is studied using thermal analysis and evolved gas analysis with on-line mass spectrometry. The cross-linked and non-cross-linked PCSs have a maximum micropore volume of 0.2 cm3 g-1 at pyrolysis temperatures of between 550 and 600 °C. The microporosity is stable in air at room temperature, but is lost in oxidising atmospheres at elevated temperatures.