By Laura Kuenssberg
BBC News political correspondent
So what is this? Is it really a "tough new crackdown" to rid the Conservatives of sleaze forever?
Mr Cameron says most Tory MEPs will sign up to the changes
Or a bit of a brush-up to distract attention from their ongoing woes with expenses and allowances? And will the Conservative party leadership be able to tame its MEPs?
Lots of questions, but no real clear answer.
The new code for Conservative MEPs will mean that, in future, they will have to give a more detailed breakdown of the amounts of money they have spent on travel, office allowances, who works for them and how much they are paid.
This is further than the Tories went in April when they announced a shake-up of their rules.
Neil O'Brien, of the lobby group Open Europe, says that the proposals do go further than any of the other parties' have gone as yet - although Labour MEPs have been subject to an independent audit since 2000 and the Lib Dems are expected to give proposals for their MEPs' allowances next week.
But the new rules will not necessarily change the behaviour of the Tories' existing brood of representatives in Brussels.
The cases of Giles Chichester and Den Dover have recently brought the expenses regime into disrepute.
And the announcement by Mr Cameron should stop that happening in the future.
To stand again for the Tories at the European elections next year, the central party says, they will have to sign up to the new code.
But making them do that may be easier said than done.
There has quite clearly been an almighty row in Brussels over the new regime, described today by Mr Cameron as "robust exchanges".
Just hours after his announcement, a leaked memo found its way to the BBC that, in black and white, displayed the disquiet among Conservatives in Brussels.
The memo criticises Mr Cameron's proposals for disclosure of MEPs' allowances, saying that the attempt to merge the European and Westminster systems "threatens the worst of both worlds".
It goes on to describe the proposal as a "direct breach of the Rules of the European Parliament".
Mr Cameron may have hoped to kill off the connection between his party and sleaze - described today by the Lib Dem Norman Baker as natural as the "link between cheese and sandwich".
But with dissent in Brussels - and the pending judgement on his party chairman, Caroline Spelman, under investigation for allegedly using her parliamentary allowance to pay her nanny - try as he might, the Conservative leader cannot shake off all vestiges just yet.