sed command

From: cent (centREMOVE_at_u.washington.edu)
Date: 12/30/04 

Date: Thu, 30 Dec 2004 07:33:18 -0800

How can I specify characters by their hexadecimal equivalent when using the
sed command?

    sed -e "s/aa/bb/g" # will replace aa with bb

I need to:

    sed -e "s/aa/ hexadecimal /g"

I have come up empty after much web searching and empirical testing. Your
help is appreciated.

Thanks.