The chitin and chitosan are polysaccharides and abundant resources of biomass, which draw attention through their distinctive biological and physical-chemical characteristics. The chitosan is a linear polysaccharide. Commercially, the chitosan is produced through the deacetylation of chitin. In order to explore the potential of these polysaccharide bio-polymers we have identified the particularities in infrared of chitin (absorbance vs. wave number), with the purpose of emphasizing the analytical possibilities of characterizing them. In this purpose, the absorption spectrum of chitin was studied in ultraviolet and mid-infrared field. The record of absorption spectrums in these favourable conditions and the processing of results, allows the quantitative expression of the acetylation degree of the amines in the polysaccharide structure. This study showed that the integral intensities of the infrared spectrum of normalized overlapped images of " amide I " straps in the case of standard mixes show a linear dependence of acetylation degree with Regression lines y= 3.716499x+0.444500 and r–squared value R 2 =0.995747; and on the grounds of the standard line and following an analytical procedure similar to the phases of standard mixes preparation, the residual acetylation degree of the real samples of chitosan can be determined.