AJAX call - weird special character problem

I'm using a form_remote_tag with a collection:

<%= select_tag "fields[]",
    options_from_collection_for_select(@fields, :keyname, :keyname),
    :multiple => true,
    :size => "10"%>

which works fine so far. The content of @fields looks something like
this:
MC1_OS_CPU_%iowait
MC1_OS_CPU_%idle
MC1_OS_CPU_%user
[...]

When submitting the form with a single value selected from the select
field, everything works fine. params:"fields"=>["BCURD01_OS_CPU_
%user"]

But when selecting multiple values, the params look like this:
fields"=>["MC1_OS_CPU_%2525idle", "MC1_OS_CPU_%2525iowait",
"MC1_OS_CPU_%2525user"]

Notice the 2525s. This only occours with special chars like "%" or "&"
inside the values.

Any ideas how to fix this?

Thanks,
Thomas

It seems to be a prototype.js issue. The POST request looks as
follows:
fields%5B%5D=BCURD01_OS_CPU_%252525system&fields%5B%5D=BCURD01_OS_CPU_
%252525user

So looks like it's a bad idea to have %-signs inside the strings.
Putting ' ' around the strings didn't help.

But there must be a way to escape them or something?