The influence of oak wood age on the crystallinity index of cellulose was determined. Antique wood originating from Warsaw woodwork from the middle of the 18th century, the turn of XIXth and 20th century and the middle of the 20th century was analysed. It was reported, using FTIR spectrometry, that the crystallinity index increases with wood age. It is consistent with previous examinations in other papers and shows that the chosen method of crystallinity index analysis is correct.