Iodine values (IVs) of some biodiesel samples were analyzed using both 1H NMR and a standard iodometric method. 1H NMR was carried out with and without 1,4-dioxane as an internal standard (IS). The results obtained using 1H NMR with 1,4-dioxane as an internal standard showed better linear fitting and correlation with the iodometric method than NMR without the IS. The proposed 1H NMR_IS method allows determination of the IV using smaller samples than the iodometric method. Moreover, the determination can be carried out in less than 15 min, dramatically less than the time needed to carry out the iodometric method. ; Authors are grateful to the Ministerio de Medio Ambiente (Project Ref.: A323/2007/3-02.5) for the financial support. The authors are also grateful to Mr. Rafael Prims from Raluy S.L. for the biodiesel supply and to Albert Tomàs and Montserra Llovera for the IV determination by the iodometric method and for the acidic profile analyses