Fibres used in preforms of resin transfer moulded (RTM) composites are coated with sizings, binders, and/or finishes that serve multiple purposes, including facilitating handling, protection of the fibres from compaction and process induced damage (including notching), aiding in compatibility and wetting of the fibres by the resin, and overall enhancement of the behavioural response of the composites. In this investigation four different sizings applied to S2 glass fibres are shown to significantly affect two aspects of RTM processing - resin infusion, and cure. In both cases phenomena at the microscopic level are seen to affect response variables at the macroscopic level. On a microscopic level, the behaviour of a thermosetting resin based composite is affected by the formation of interphase regions that greatly affect the cure kinetics and hence the mechanical and physical properties of the composite, which are dependent on the inter-constituent variations in local properties such as modulus and glass transition temperature. Similarly fibre-sizing-resin interactions occurring during the infusion stage affect wet-out and local flow behaviour through the development of stoichiometric imbalances in local regions. It is shown that the molecular interactions between the constituents (as initiated by the sizing) are affected by processing conditions such as temperature and rate of resin flow, and that heat evolution and resin rheology may be affected by the stoichiometric imbalances resulting from interphasial level reactions.