I have 'Option ZAxisMapping "4 5" ' set in XF86Config-4, but the system doesn't recognize the scroll wheel of my Logitech optical mouse (it recognizes it as a third button but not as a scroll wheel). When I run xev I get no reaction to scrolling the wheel.

It might help to tell us what kind of mouse it is exactly (model and make). Also what distribution of linux you're running, or at least kernel version.

Seems to me that if xev doesn't pick up that you're moving the scroll mouse then you need a special driver for it. I ran into the same problem with my Microsoft multimedia keyboard. There are four keys on it that show nothing in xev so I have been unable to use them.

Look for support on your mouse specifially on the internet. Or post a little more information and maybe someone else can tell you more.

if it's a ps/2 mouse you have to say it's of type "ImPS/2"
and i think you might need to add
option "Buttons" "5"

I'm not in Linux right now, so syntax may be wrong

Don't know if it matters, but you wrote:

'Option ZAxisMapping "4 5" '

maybe it must be

'Option "ZAxisMapping" "4 5" '

I have it alway this way, and it worked... but as I said, don't know for sure

Thanks all for the help; changing 'PS/2' to 'ImPS/2' seems to have done the trick. I wonder why the Debian autoconfig doesn't get that right? And what is the difference between the two?