Hydrogen absorption in LaNi5 intermetallic compound has been studied by in situ synchrotron powder diffraction. Fast data acquisition allows the measurement to be performed under dynamical conditions, i.e., while the full reaction proceeds. The major finding of this study is evidence of a non-equilibrium transient γ phase never reported at room temperature without prior high-temperature cycling. Other findings are the variations of the extent of the line broadening and changes in the line shape during the absorption and desorption processes. They are explained by a reorganization of the hydrogen-induced dislocations in α and β phases.