The lithium insertion in the ordered olivine-type structure of LiFePO4 was analyzed as an insertion process with a Frumkin-type sorption isotherm. A minimum in the chemical diffusion coefficient of lithium (DLi) was predicted by the model for strong attractive interactions between the intercalation species and the host matrix. The DLi in the material was measured as a function of the lithium content by using the galvanostatic intermittent titration technique (GITT). The diffusion coefficient was found 1.8 x 10(-14) and 2.2 x 10(-16)cm(2) s(-1) for LiFePO4 and FePO4, respectively, with a minimum in correspondence of the peak of the differential capacity. The DLi has also been measured by AC impedance method for various lithium contents. The calculated values are in very good agreement with the previous calculated ones.