The motivation behind this study was primarily commercial---in the brick industry the colour of the finished article is a very significant commercial factor. Initially a series of mineralogical analyses, selective chemical extraction procedures and firing trials were undertaken to investigate and identify the potential colour-inducing constituents of fired and unfired brick clays. Iron-rich clay minerals were identified as a major source of colour in the fired bricks. This study then concentrated on the investigation of the role of iron and related colour-inducing transition metals in the thermal reaction sequence(s) of fired clays primarily by means of X-ray Diffraction and Transmission Electron Microscopy. Electron Microscopy identified iron oxide (Fe2O3) as the primary coloured phase in fired brick clays and successfully correlated changes in colour with firing temperature with changes in oxide crystallite size and composition. Some inconsistencies were evident between the results of this study and the previously accepted mechanism behind the high temperature transition series of layer silicates. Consequently, in line with an additional aim of identifying the precise reaction mechanism(s) responsible for the transition series and the development of the coloured phases, an alternative hypothesis is put forward to cover the (lower temperature) transitions.