One of the most often methods of intermediate product stabilization is their cross-linking. Aldehydes are commonly used as cross-linking chemicals, from which glutaraldehyde and formaldehyde have the greatest importance. Final quality and optimal behaviour of the end product depend on various conditions during the cross-linking reaction. This book deals with mathematical description of cross-linking process represented by the nonlinear vector differential equations. Linearization of the equations is given to determine a transfer matrix. In the experimental part, the physical methods exploiting changes of electrical and optical properties of investigated polymer are used. These changes allowed accomplishing not only experimental identification of the cross-linking system but also partial clarification of the first-step mechanism when poly-electrolytic circle of the cross-linking protein is opening. Kinetic curves acquired by the above-mentioned physical measurements are presented.At the end of this work, the results that can be applied in practice during natural polymer treatment are discussed.