  We use electron momentum density in ice as a tool to quantify order-disorder
transitions by comparing Compton profiles differences of ice VI, VII, VIII and
XII with respect to ice Ih. Quantitative agreement is found between theory and
experiment for ice VIII, which is the most ordered phase. Robust signatures of
the oxygen disorder are identified in the momentum density for the VIII-VII ice
phase transition. The unique aspect of this work is the determination of the
fraction n_e of electron directly involved in phase transitions as well as the
use of position space signatures for quantifying oxygen site disorder.
