Cellulose is the most abundant biopolymer in the world – cellulose originating from waste textiles has the potential to be recycled into valuable future applications. Cellulose nanocrystals, CNCs, are the crystalline regions of cellulose fibrils and can be separated thorough acid hydrolysis. In this study, CNCs were produced from a cotton t-shirt using a new method of hydrolysis using hydrochloric acid gas. The addition of an alkaline pre-treatment significantly decreased the final degree of polymerization of the fibres by improving the reactivity and degrading the hemicellulose and lignin also present in cotton. Future research and characterization of the produced CNCs is required to determine the suitability of cotton as a raw material for commercial CNC-based products.