Matrix-assisted laser desorption/ionisation mass spectrometry time of flight (MALDI-TOF) was used to estimate the average molecular weight of PEGs (polyethylene glycols). Only the molecular weights determined by the reflectron mode were used because of better resolution. The average molecular weights were: PEG 1500 (1638), 4000 (4129), 6000 (6343), 10,000 (12,774) and 35,000 (34,650). The melting behaviour of PEGs in the molecular weight range 1500-35,000 have been examined. Infrared spectra showed the appearance of a new weak band at 1326 cm-1 for PEG 12,774 which increased in strength with increasing molecular weight suggesting an increase in amorphous content. This was confirmed by differential scanning calorimetry (DSC) and X-ray powder diffraction (XPD). The decrease in the enthalpy of fusion and the degree of crystallinity above molecular weight 6343 suggested maximum crystallinity at this molecular weight. Generally untreated and slow cooled PEG exhibited higher enthalpies of fusion than quench cooled, suggesting that they are more crystalline. Scanning electron microscopy (SEM) showed a more pronounced spherulitic structure for untreated and slow cooled PEG 12,774, while small or imperfect spherulites were observed after quench cooling. This was supported by XPD. The crystallinity for PEG 12 774 increased from quench cooled (71.5%), < untreated (78.1%) to < slow cooled (81.5%). The enthalpies of fusion of untreated PEGs were higher than slow cooled PEG in the molecular weight range 1638-6343, while the reverse occurred for PEG 12,774-34,650. These PEGs exhibited a higher proportion of amorphous material initially as suggested by IR spectra and an increase in crystal perfection may occur during storage.