The structure of Li1-xNiO2 is studied as Li is electrochemically deintercalated from LiNiO2 with in situ X-ray diffraction methods. The electrochemical behavior of Li1-xNiO2 is investigated during the X-ray diffraction measurement and in other cells. We find three hexagonal phases and one monoclinic phase within the composition range 0.0 less than or equal to x less than or equal to 0.82. Three regions of x corresponding to two-phase coexistence are found by X-ray diffraction. The electrochemical measurements show strong peaks in dx/DV in these coexisting phase regions in agreement with the X-ray data. A phase diagram for Li1-xNiO2 as a function of x is presented. A new in situ X-ray cell design, suitable for reliable operation with high voltage cathodes like Li1-xNiO2, is also described. Comparison to other recent work is made.