The reaction of tetrahydrofuran (THF) with KC 24 and RbC 24 graphite intercalation compounds has been studied in situ by neutron diffraction measurements. The patterns obtained during intercalation and deintercalation of the organic molecules were recorded on a time scale of two minutes/spectrum using the DIB multidetector at I.L.L. (Figs. 1 and 2). We have successively observed the formation of two first stage ternary phases: MC 24 (THF) 1 ( M = K, Rb) where the THF molecules lie parallel to the graphitic planes (//), then the richer ternary phase MC 24 (THF) 2 in which the organic molecules stay perpendicular to the planes (⊥). By cryopumping, MC 24 (THF) 2 (⊥) can be reversibly transformed into MC 24 (THF) 1 (//). The analysis of the (100) graphite diffraction peak shows a shift to the lower theta during the THF intercalation into the binary compounds indicating an increase of d c- c with the ternary compound formation. This increase, related to a charge transfer from the intercalate to the graphite layers, is more important with the potassium ( Δd = 0.007 A ̊ ) than with the rubidium ( Δd = 0.004 A ̊ ) like it has been previously observed during benzene intercalation. d c- c remains quite constant in the two ternary phases (// and ⊥). Owing to the scattering cross-section of the deuterium comparable to those of carbon and oxygen, the structural analysis of the (00 f) neutron lines provides more informations than X-ray diffraction measurements can do. So, we have been able to fit all the (00 f) intensities observed and calculated with different ratios m = THF/ MC 24. The better agreement was found with m 1 = 1.6 for the (//) phase and m 2 = 2.45 for the (⊥) one. This result confirms that the intercalation number m is limited by the free space between two adjacent graphitic planes. On the other hand, m 2 was calculated with two hypotheses: all the molecules standing parallel to the c axis with the oxygen atom leveling at the same graphite plane (A model) or half leveling at a plane and half at the adjacent one (B model). The cristallographic results can only be fitted to the A model (Fig. 6). The in situ structural observation during intercalation revealed that the THF action on KC 24 and RbC 24 second stage binary compounds leads to first stage ternary compounds via intermediary second stages ternary phases. Nevertheless, the second stage phases diffract with a weak intensity and during a relatively short time range. Furthermore, and for the first time, it has been observed the appearance of the first stage binary compound MC −8 during the first stage ternary MC 24 (THF) 1.6 (//) formation, then its decrease with the transformation MC 24 (THF) 1.6 (//) → MC 24 (THF) 2.45 (⊥). During cyropumping, the formation of the (//) phase from the (⊥) one goes again with the MC −8 growth (Fig. 3). Such a mechanism can be explained by the alkali atoms reorganisation inside the graphite layers: the bulky organic molecules penetration induces a back flow for some alkali atoms which involves the growth of the binary domains MC −8 in the same time that the ternary domains formation. This necessarily implies a ratio M C weaker in the ternary phase M x1 C 24 (THF) 1.6 (//) than in the initial binary compound M x0 C 24 ( x1 < x0). Furthermore, the intensity disappearance for the (00 f) diffraction peaks related to the RbC −8 phase while the (//) phase is transformed into the (⊥) one: Rb x2 C 24 (THF) 2.45 indicates that X2 is close to X0. So, in the ternary compounds as in the binary ones, the ratio C/ M = 24 is an ideal integer value which may vary from one phase to another. Our results tend to a ratio THF/ M = 2 in agreement with EXAFS measurements repons where it was found only one distance M-0 in the two phases (// and ⊥), and the same oxygen number around the alkali atoms[20]. This observation is consistent with the two solvating THF molecules lying parallel to the graphitic sheets before straightening and becoming perpendicular when the THF pressure increases. In this hypothesis, the phases would be represented by MC 30 (THF) 2 (//) and MC 20 (THF) 2 (⊥) where x2/ C − 1 20 close to the ratio x0/ C in the initial binary, agrees with the existence of several binary phases with different stoichiometries[24].