Thanks for the reply.

Yes, I understand that is how it works. I should have provided more information.

I have the following added to the default serendipity .htaccess file:
RewriteCond %{http_host} ^www.example.com [nc]
RewriteRule ^(.*)$ http://www.example2.com/blog/$1 [r=301,nc]

It works ok ...

I am trying to get 301 redirect to work so that my old links in google at example.com/blog/... are 301'd to example2.com/blog/...

I have a .htaccess in my root directory and ofcourse the one in the serendipity directory (/blog). Which should I edit?

I currently have the following in my example.com ...