Liberal Democrat leader Sir Menzies Campbell has paid tribute to outgoing Prime Minister Tony Blair for being "unfailingly courteous".
Sir Menzies said he and Mr Blair had had several disagreements
In Mr Blair's final prime minister's questions, Sir Menzies also conveyed his "best wishes" for the future.
Mr Blair returned the compliment, saying Sir Menzies had a "generosity of spirit and courtesy".
On Tuesday, Sir Menzies threatened to "rattle the cage of British politics" after Gordon Brown replaced Mr Blair.
In the Commons on Wednesday, Sir Menzies said: "As the prime minister knows, he and I have had a number disagreements, not least upon Iraq.
"But in relation to our personal dealings, he has been unfailingly courteous and I would like to express my gratitude to him for that.
"As he leaves office, may I, on behalf of my colleagues, extend our very best wishes to him and his family."
Mr Blair replied: "I thank him for that and let me return the compliment to him, because I think, whatever differences we have had politically, I don't think anybody in this house would think that he is a person other than one of generosity of spirit and courtesy.
Sir Menzies, a long-time friend of Mr Brown, has promised to "scrutinise his policies with relentless, forensic vigour".
He has also dismissed the chance of Lib Dems being in a Labour Cabinet, saying there was a "chasm" between parties.
Last week former Lib Dem leader Lord Ashdown said he had been offered the Northern Ireland secretary job after Sir Menzies had said no member of his party would join Mr Brown's government.
But on Sunday Mr Brown said: "This is not a correct account of what happened. It is a total travesty of what happened."