An assessment of stepped heating procedures for the extraction and isolation of carbonaceous species from fluid inclusions resulted in the development of low-blank procedures which permitted Sl3C characterisation of palreofluid CO2 (down to nanomole quantities) with an accuracy approaching that of the corresponding analytical precision. Similar procedures were successfully applied to the ol5N measurement of palreofluid nitrogen at the sub-nanomole level. An investigation into the origin of fluids which characterised the earliest episodes of palreohydrothermal activity associated with the granites of S W England indicates that the abundance of trace carbon species (C02 , CH4 ) and nitrogen in the fluids was correlated with the metasedimentary contribution to the respective granite source. Furthermore, SiSN and ol3C data (obtained on fluid components and local Palreozoic metasediments, in conjunction with published ol5N values of Cornubian granites), indicate that carbon and nitrogen in the hydrothermal systems were derived from the granite magmas. The chemical composition of the early hydrothermal fluids, together with geochemical and isotopic constraints from the characterisation of Palreozoic metasedimentary country rocks, support the view that the fluids were genetically associated with the granites. Fluid interaction with the local metasedimentary rocks at a high level crustal appears to have been very limited. The incorporation of sedimentary matter into granitic protoliths during anatexis, with subsequent transfer to an exsolved hydrous phase during pluton cooling, is the most probable route by which palreofluid solutes entered the early hydrothermal systems. Hydrogen stable isotope data, measured on the extracted palreowaters, indicate that meteoric water was not a significant component of early hydrothermal systems associated with either the Dartmoor granite or the nearby Hemerdon Ball intrusive, if sub-solidus isotopic exchange was significant. In contrast, comparable data from early fluids associated with other component intrusives of the batholith (as characterised by W ± Sn oxide paragenesis) are consistent with the progressive dilution of a magmatic-hydrothermal component by local groundwaters.