Deamidation of glutamine (Q) and asparagine (N) has been recognized as a marker of degradation and aging in ancient proteins. Using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-ToF-MS) to study deamidation in wool textiles, we identified eight peptides from α-keratin proteins in sheep wool that could potentially be used to assess the level of degradation. For each chosen peptide, the extent of deamidation was determined by comparing the calculated theoretical distribution with the measured distribution using a genetic algorithm that gives the best fit to the measured distribution. Variations in the levels of deamidation were observed between peptides and in modern wool samples buried for up to 8 years in which deamidation levels were relatively low under short-term burial. In contrast, deamidation was higher in archaeological textile fragments from medieval sites ranging from the 9th to 13th c. in York (UK) and Newcastle (UK) and from the 13th to 16th c. in Reykholt (Iceland). Major differences were observed between the British and the Icelandic samples, showing a negative correlation between age of samples and levels of deamidation, but highlighting the effect of local environment. In addition, nanoLC-ESI-MS/MS data indicated that deamidation in wool's α-keratin was influenced by primary and higher-order structures. Predominance of deamidation on glutamine rather than asparagine in the archaeological samples was attributed to a higher abundance of Q in the alpha-helical core domain of keratins, neighboring residues and steric hindrance preventing deamidation of N.