Ion composition variations in the inner magnetosphere during storm times are studied by using data sets obtained from the magnetospheric ion composition spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES). The observations were made during the second half of the CRRES mission, which was near the maximum of solar cycle 22. Four selected storms are subjects of detailed case studies; statistical results are based on a group of moderate (50100 nT) storms observed in 1991. The case studies show that energetic particle enhancements occur at very low equatorial altitudes (L=3~4) during large storms with significant delays relative to the storm sudden commencement times (of about 20 hours). The average time duration of the particle enhancements is about 47 hours. By studying the time variation of energy spectrograms of H+, it is found that low-energy (E100 keV) protons show different time profiles during the development and decay of the ring current. The low-energy part shows a dramatic intensification and a rapid decay. However, its relative contribution to the ring current defined by the density ratio N(HL+)/N during the storm maximum is almost constant. On the other hand, high-energy protons first exhibit a flux decrease followed by a delayed increase. The density ratio N(HH+)/N shows an anticorrelation with the storm intensity. It is confirmed that the ionospheric origin particles (e.g., O+) are important constituents of the storm time ring current. The fractional number density of O+ ions increases with the intensity of the storm. The statistical results demonstrate that the energy density of O+ is a steep function of Dst for moderate storms. However, it seems to increase very slowly with Dst, or even to be almost independent of Dst for large storms (|Dst|≥120 nT). The ratios of solar wind origin He++ density to the total density show no obvious difference among large storms. The same appears for He+ ions. ¿ 2001 American Geophysical Union