Compose key in LXDE?

setxkbmap -option "compose:caps" for CapsLock

setxkbmap -option "compose:ralt" for right alt =)

To set the compose key

sudo leafpad /etc/default/keyboard

edit XKBOPTIONS to like this

XKBOPTIONS="compose:Shift+AltGr"   

To get a list of available keys:

grep compose /usr/share/X11/xkb/rules/xorg.lst

Reboot for the change to be applied or

setxkbmap -option "compose:Shift+AltGr"

To check if the option was set correctly

setxkbmap -print -verbose 10

Example:

press Shift then AltGr and release then click on "o c" to obtain ©.

Additional reading

• ArchLinux wiki - Keyboard configuration in Xorg
• Duncan Lock's blog - How to set your Compose Key on XFCE/Xubuntu Linux

You can check if the keyscan code on your keyboard same as default setting. Did you check if you can do the compose by selecting predefined keyboard with Compose key?