There are dozens of different X5 routes across the country.
The one which immediately comes to mind for me is the Oxford to Cambridge service, operated by Stagecoach. On that service the answer to your question is 'Yes':
http://www.stagecoachx5.com/faqs.aspx#9
Eddie51's statement that passes are not accepted on airport services is not always correct. A couple of years ago I was talking to driver of such a service (which, coincidentally, was the Ipswich to Stansted service referred to by Hopkirk) and he said that bus passes were accepted on any stage of a journey where there were less than 15 miles between the stops at either end of it. Because there was no point on that route where a boarding passenger would ever be more than 15 miles away from the next stop, passes were accepted throughout the route.
Since then, however, the Government has slightly revised the rules so that services on which the majority of seats can be reserved are no longer covered. So that driver's statement might no longer be true:
http://www.direct.gov...achTravel/DG_10036264
If you tell us which X5 service your question refers to, we'll try to provide a definitive answer (or tell you where you find out).
Chris