ION Enterprise 3.0 MSDE 1.0 (SQL Server 7.0)
ION Enterprise 4.0 MSDE 1.0 (SQL Server 7.0)

ION Enterprise 4.5 MSDE 1.0 (SQL Server 7.0)

ION Enterprise 5.0 MSDE 2000 (SQL Server 2000)

ION Enterprise 5.5 MSDE 2000 (SQL Server 2000)

NOTE: It is possible to use ION Enterprise 5.5 Service Pack 2 with SQL Server 2005, but some modifications are required. See KB articles 12704, 12708 and 12823 for detailed information.

ION Enterprise 5.6 SQL 2005 Express (SQL Server 2005)
NOTE: ION Enterprise 5.6 will use an existing MSDE 2000 instance but will install SQL Express on new systems if there is no pre-existing SQL Server instance.
ION Enterprise 6.0 SQL 2005 Express with Reporting Services SP3 (SQL Server 2005 SP3 with Reporting Services)
NOTES: The ION Enterprise 6.0 installer will upgrade an existing MSDE 2000 instance to SQL Server 2005 Express with Reporting Services. ION Enterprise 6.0 does not support SQL Server 2000. It also does not support SQL Server 2008