Single- and reflected-shock compression measurements on single-crystal ilmenite (FeTiO3) to pressures of 635 and 930 kbar, respectively, demonstrate anomalously large compressions above ~320 kbar and imply the onset of one or more major phase changes at this pressure level which continue to occur to at least ~600 kbar. The phase change onset pressure is considerably above the calculated upper bound of 193 kbar for the stability of ilmenite with respect to breakdown into wustite and rutile. Over the 300- to 600-kbar range, samples shocked along the c axis are consistently more compressible than those shocked along the perpendicular direction; at a given density the Hugoniot curves differ by 40 kbar. The observed density (6.25 g/cm3) of the highest-pressure single-shock state implies that the shock-induced high-pressure phase has a zero-pressure density of at least 5.4 g/cm3, or a density 13% greater than that of ilmenite. The reflected-shock data have a larger experimental uncertainty but indicate an even greater zero-pressure density for the high-pressure phase. The large density increase inferred restricts the possible nature of the high-pressure phase. These data are consistent with the 1300-kbar datum reported by Simakov et al. (1974) and with the disproportionment of ilmenite into mixed oxides in the rock salt (FeO) and fluorite (TiO2) phases as observed (Liu, 1975) in static recovery experiments.