Hi Ellen,
If you only wanted to redirect all the url's in a certain subdirectory to the new site (not 1-on1), your redirect would look something like this:
RedirectMatch 301 ^/subdirectory/$ http://www.websiteB.com/
But... I dont think you want to do that
(Or should do that...) I think you should want to redirect all pages in the subdirectory to their corresponding url on websiteB.
What I mean by that is: you should redirect all your pages 1-on1, like this:
Redirect: www.websiteA.com/subdirectory/url1.html to its equivalent on website B's url: www.websiteB.com/url1.html
If you want to do that, you can. But the url's have to be the same. You can do something like:
RedirectMatch 301 /subdirectory/(.*) http://www.websiteB.com/$1
If the url's change, than you have to do more manual work. You should always try to redirect as many pages 1on1 as you can. Especially the ones that receive external links. The new pages will be almost as "juicy" as the old pages, because the 301 redirect passes the linkjuice along. (not all of it though, but like 90% - 99%)
Is this pushing you a bit in the right direction? 