In this study, mercerisation of native cellulose I was achieved in an aqueous sodium hydroxide solution at a concentration of only 1 wt% NaOH by processing at temperatures below 0 °C. This represents a tenfold reduction in the use of NaOH to accomplish this very common transformation. The cellulose sample was a form of hydrolysed cotton with a high crystallinity. The samples were mixed with aqueous sodium hydroxide at various concentrations and stored at −17 °C. The samples were then defrosted, neutralised and dried before being analysed by Fourier-transform infrared spectroscopy, wide-angle X-ray diffraction and field-emission scanning electron microscopy. In the route described here, transformation from cellulose I to cellulose II was possible without greatly affecting the crystallinity or the microstructure of the samples.