This thesis examines contrasting styles of magmatic differentiation, inferred magma storage conditions and the behaviour of volatile species prior to explosive Plinian and subplinian eruptions at Mt Somma-Vesuvius, southern Italy, Two case-study events are compared: the Plinian eruption of ~3,55 kaBP ('Avellino') and the smaller, more recent subplinian eruption of 472 AD ('Pollena'), These represent opposite end members within the scheme of violently explosive major eruptions at Somma-Vesuvius, in terms of their pre-eruptive repose times, eruptive scenarios, geochemistry and stratigraphic zoning of their juvenile clasts within their fall deposits, The study employs detailed microanalysis of melt inclusions (MI) and their host crystals from the eruptive products of these two events, and compares these new data and observations with published MI and whole-rock data from these and other key eruptions of Somma-Vesuvius, Chapter 2 compares SIMS, Micro-Raman and EPMA measurements of H20 concentration in the same MI and documents the pitfalls of these approaches and the efforts made to mitigate them, In Chapter 3, new and already-published major and trace element data for MI and their host crystals are used to account for clear differences in the degree and style of magmatic (whole-rock) differentiation prior to the two case-study eruptions in terms of differing crystallising assemblages controlling liquid evolution, differing degrees of total fractional crystallisation and relative volumes of the most evolved liquid compositions in the storage system, Chapter 4 reveals that the most evolved magmatic liquids in the Avellino system apparently had higher dissolved water contents than those from Pollena. Relationships between MI H₂0 and CI contents and major and trace elements indicate that magmatic liquids reached saturation with both vapour and brine phases prior to both eruptions and that significant isobaric magmatic differentiation occurred under these volatile-saturated conditions, Maximum H₂O contents and the stability of leucite in Pollena magmas, compared with its total absence from A vellino, are both consistent with shallower storage of the most evolved magmas at around ~ 1 kbar total pressure, compared to ~ 2 kbar for Avellino. A detailed study of sanidine-hyalophane crystals, complex zoning in highly Baand Sr-rich variants, and the relationship between entrapped MI and host-crystal compositions reveals drastic differences in the crystallisation environments recorded by Pollena versus Avellino crystals (Chapter 5). These differences, together with those in bulk-magmatic differentiation and dissolved volatile contents, are interpreted as indications of a relatively mature, deeper and larger volume storage system of highly evolved phonolitic magmas in the case of Avellino, compared to a relatively immature, shallower, complex and smaller volume equivalent for Pollena, Finally an exploratory study (Chapter 5) of sector zoning of Ca, Sr, Ba and Fe in Pollena sanidine-hyalophane crystals implies that, far from being a kinetically controlled disequilibrium feature, there is an intrinsic thermodynamic system control on the strength of apparent intersector partitioning. Temperature is proposed as the most likely control in this case, and the possibility is raised of using this phenomenon for single-crystal thermometry in the future.