To get basic information on the complete actinide series radiochemical measurements are necessary. Since the structure of the aqueous trivalent ions is not known, we undertook a study of transport properties. These properties, like limit diffusion coefficient, are related to the size of the hydrated ion and therefore its variations along the 5f series should give informations on discontinuities in the size of the aquo ion and finally the number of water molecules surrounding the central ion. Data have been obtained through a comparison of 4f and 5f elements with the open and capillary method. Aqueous solutions were studied at 25.00 °C, pH 2.5, with LiCl or Nd(ClO 4) 3 as supporting electrolyte. For each investigated element, a linear variation of D versus√ c is observed in the LiCl concentration range 4 × 10 −3 M < c < 5 × 10 −2 M. After correction from the contribution of HCl, limit diffusion of coefficients D° are obtained, for the studied lanthanides: Ce: 6.20, Gd: 5.74, Tb: 5.79, Tm: 5.80, Yb: 5.78 × 10) −6 cm 2 s −1 and for trivalent actinides: Am: 6.25, Cm: 6.11, Cf: 5.87 and Es: 5.77 × 10 −6 cm 2s −1. The S shaped changed in the overall hydration of the cation across the considered series is correlated to the changed in the primary hydration number N. Using the variation of N versus the crystallographic radius of the lanthanide and diffusion coefficient data for the actinides, we estimated N for each trivalent ion of the actinide series. A change of N from 9 to 8 should occur between Bk and Cf. Moreover, since entropy of trivalent lanthanide ions depends on their structure we obtained a new determination of the entropy of the An 3+ aqueous ions. Finally we calculated hydration enthalphy of trivalent actinide ions with the same analytical expression we established for 67 monovalent, divalent, trivalent and tetravalent ions.