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The subject of this thesis is to adjust the destructive method of radiocarbon dating and delimiting the sample size in radiocarbon dating of medieval manuscripts made from parchment. Manuscripts are very valuable and as a material can be very scarce, and considering that sampling manuscripts has to be minimally destructive, delineating the sample size is essential. In radiocarbon dating the usual amount of carbon needed for Accelerator Mass Spectrometric (AMS) analysis is 1 mg, and as the experimental part of this thesis, series of tests were concluded in order to test whether taking down the amount of carbon towards small samples with <1mg carbon is possible, as well as determine the smallest sample size in dating parchment. As in small sample sizes the amount of contamination becomes more operant, a method was developed in order to remove all possible contamination: a chemical pretreatment process and moving the process to a clean room space, targeting in removing both modern and old carbon contamination possibly present in the sample. Modern carbon contamination can derive from conservation treatments or through handling of the parchment, and old carbon contamination might be present from the original manufacturing process or use of the parchment. Due to various origins of contamination, the technique, history and conservation of parchment are addressed carefully, are they affect the method developed in the study. Altogether three parchments with known age were used in this study, from the 21st and 15-16th centuries. Based on the test-runs made, the sample size can be taken down from 1 mg of carbon to 0,7-0,5 mg of carbon for radiocarbon dating and the size of the samples was defined successfully. As a result a new clean room based method was developed which will be used in the upcoming research and dating of the parchment fragments from the Fragmenta membranea collection in the National Library of Helsinki.