Symbian OS v8.0. First shipped in 2004, one of its advantages would have been a choice of two different kernels (EKA1 or EKA2). However, the EKA2 kernel version did not ship until SymbianOS v8.1b. The kernels behave more or less identically from user-side, but are internally very different. EKA1 was chosen by some manufacturers to maintain compatibility with old device drivers, whilst EKA2 offered advantages such as a hard real-time capability.

Symbian OS v8.1. Basically a cleaned-up version of 8.0, this was available in 8.1a and 8.1b versions, with EKA1 and EKA2 kernels respectively. The 8.1b version, with EKA2's single-chip phone support but no additional security layer, was popular among Japanese phone companies desiring the realtime support but not allowing open application installation.

Symbian OS v9.0. This version was used for internal Symbian purposes only. It was deproductized in 2004.

Symbian OS v9.1. Early in 2005, the newest version of Symbian was announced. Improvements in the OS mean that applications and content, and therefore a developers investment, are better protected than ever. The new ARM EABI binary model means developers need to retool and the security changes mean they may have to recode. The Nokia N91 will probably be the first SymbianOS 9.1 device on the market, though the Sony Ericsson P990 should follow closely.

Symbian OS has generally maintained reasonable binary compatibility. In theory the OS was BC from ER1-ER5, then from 6.0 to 8.1b. Substantial changes were needed for 9.0, related to tools and security, but this should be a one-off event. The move from requiring ARMv4 to requiring ARMv5 did not break backwards compatibility.