Raman spectroscopy has been used to follow the hydration of one of the principal components of Ordinary Portland Cement (OPC) clinkers, tricalcium aluminate, both in the absence and in the presence of calcium sulfate. Direct in-situ analysis of the hydrating paste surface was possible. Spectra were recorded regularly during the first 24 hours of hydration, and then systematically after hydration periods of up to 28 days. X-Ray diffraction was performed to confirm sample identity, firstly after 24 hours and then regularly in coordination with Raman analysis. Hydration in the absence of sulfate was rapid and led to the initial formation of C₄AH₁₉ and subsequently C₃AH₆. Characteristic changes were seen in the Raman spectra and the products' identities confirmed by XRD. Hydration in the presence of sulfate led to very rapid ettringite (AFt) formation, with the first evidence of ettringite formation within 3 minutes. Ettringite gradually converted to monosulfate (AFm), with the re-formation of traces of gypsum at high sulfate contents. Ettringite was formed at the expense of C₄AH₁₉. The level of C₄AH₁₉ formed diminished greatly with increasing sulfate content. Care was taken throughout the study to exclude carbon dioxide, thus minimising formation of various carbonate species, which could also be readily distinguished by Raman spectroscopy. By combining the results obtained using the well-established technique of X-ray diffraction with Raman spectroscopy, in-situ characterisation of hydrating pastes has been achieved in real-time. In addition to demonstrating the suitability of Raman spectroscopy for analysis of these systems this study should aid the understanding of tricalcium aluminate hydration and the formation of ettringite and monosulfate.