A method of preparing a heparin derivative having an average molecular weight of about 4.6 to about 6.9 kDa and an anti-factor Xa activity of less than about 10 IU / mg, and wherein the heparin derivative has a disaccharide that is predominantly produced as shown in the following formula I ** Formula ** in which ** Formula ** and n is an integer from 2 to 20, corresponding to molecular weights between 1.2 and 12 kDa, and wherein the heparin derivative has signals in the region of 5.0 ppm to 6.5 ppm of a 1 H NMR spectrum with an intensity (proportion in%) less than or equal to about 4% relative to the signal at 5.42 ppm of the 1 H NMR spectrum of unfractionated heparin, which comprises the consecutive steps of: (i) oxidizing an acidic aqueous solution of unfractionated heparin by the addition of an oxidizing agent； (ii) depolymerize the oxidized heparin by subjecting the product of step (i) to an alkali to form an alkaline solution； (iii) maintaining said solution of step (ii) at an alkaline pH for a period of time required to provide depolymerized heparin with a molecular weight within the aforementioned range； and (iv) reducing the aldehyde end groups of said depolymerized heparin by adding a hydride reducing agent to the solution obtained in step (iii)； wherein the period of time between the end of stage (i) and the beginning of stage (iv) is controlled in order to minimize the effect of residual oxidizing agents； and wherein said time period of stage (iii) is determined by analysis of said solution or by reference to a substantially identical stage (iii) carried out previously.Un procedimiento de preparación de un derivado de heparina que tiene un peso molecular promedio de aproximadamente 4,6 a aproximadamente 6,9 kDa y una actividad anti-factor Xa inferior a aproximadamente 10 UI/mg, y en el que el derivado de heparina tiene un disacárido que se produce predominantemente tal como se muestra en la fórmula I siguiente**Fórmula** en la que**Fórmula** y n es un núm
