J. R. Dwyer, G. M. Mason, M. I. Desai
	University of Maryland, College Park, Maryland, USA 
J. E. Mazur
	Aerospace Corp., El Segundo, California, USA

Since its August 1997 launch, ACE/ULEIS has observed several energetic particle enhancements at 1 AU associated with corotating interaction regions (CIRs). These CIR events are distinguished from the more common solar energetic particle (SEP) events by sunward anisotropies, high C/O ratios (C/O ~ 1) and the occurrence of high speed solar wind streams. With the large geometric factor and excellent mass resolution of ULEIS, we have measured the time and energy dependence of the ion composition for four CIRs in 1998 and 1999. These events all have Mg/O and Fe/O ratios that are consistent with a mixture of fast and slow solar winds. However, they also have enhanced C/O and Ne/O values with respect to the solar wind. While these results agree with earlier measurements made by the WIND/STEP instrument, ULEIS also measured elemental and isotopic abundances such as 3He/4He, N/O, Si/O, and S/O, which were not possible with earlier instruments. We will present new observations of energetic particles originating from CIRs and discuss implications for the source materials and acceleration mechanisms involved.