Water diffusion through low density polyethylene (LDPE) film was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and two-dimensional correlation spectroscopy (2DCOS) over a temperature range of 25-80 degrees C. The 2DCOS spectra of water diffusion in LDPE at different temperatures were very similar. The broad OH stretching vibration (v(OH)) was split into four bands assigned to three different types of water molecules, namely, bulk water, cluster water, and free water, which will be defined. It appeared that as water molecules disperse in LDPE, cluster water with moderate hydrogen bonds diffused faster than bulk water with strong hydrogen bonds. Additionally, the hydrogen bonds between water molecules may have been further weakened or even broken to form free water, possibly due to limited free volume. On the basis of a Fickian model, the diffusion coefficients could be calculated from overall, fast and slow diffusion process, respectively. The diffusion coefficients were observed to increase with temperature, possibly due to increased swelling of molecular chain, more free volume, and reduced size of the penetrants.