Since the introduction of the injection stretch blow moulding (ISBM) process, poly(ethylene terephthalate) (PET) has been the preferred material for the formation of thin-walled, light-weight containers. PET preforms injection moulded are supplied to the end user bottle manufacturer; this may be a direct link from injection moulding phase to the ISBM reheat phase, or the preform may remain in storage for an indefinite amount of time. As PET can absorb moisture form the environment (hygroscopic), the amount of time the preform is stored for may become critical. This paper investigates the effect of moisture content on the thermal properties of PET and the resultant influence on the deformation behaviour. Removing the mould and performing a free-stretch-blow analysis, the internal cavity pressure and stretch-rod reaction force was recorded with an instrumented stretch-rod and the surface strain was observed using a non-contact, stereoscopic digital image correlation (DIC) technique. Thermal material transition phases as a function of moisture were investigated using differential scanning calorimetry (DSC) analysis. Results reveal that the moisture content can influence the material deformation behaviour and effectively shift the glass transition temperature and onset of thermal crystallisation. The effects of moisture level on the processability of the PET preform will undoubtedly have an influence on the formation of the bottle during ISBM and as a result the material distribution and mechanical properties of the final bottle. This knowledge is critical at various stages in PET bottle supply chain from material suppliers and preform manufacturers to converter and end users. Although there are strict criteria for handling PET granules during the preform injection moulding phase, there is currently no specific standard for appropriate preform storage during bottle manufacturing.