Mr Kennedy, who lost his seat at May’s general election after 32 years as an MP, died at his home on Monday.
Former Scottish Lib Dem leader Tavish Scott said despite the defeat Mr Kennedy had been upbeat.
“He was in fantastic form and talking about taking a leading role – if asked – in the EU referendum campaign, as well as thinking about standing for Holyrood next year.”
Former Edinburgh West Liberal Democrat MP John Barrett said he was “shocked and stunned” by news of the death.
He said: “My proudest moment as an MP was being united as a party in opposition to the Iraq war, led by Charles.
“Lots of people now look back and say it was wrong to go to war, but Charles got a lot of abuse for the stand we took.”
Mr Barrett added: “He had his problem with drink and people knew that. Charles always said ‘I’m a Scotsman, a Highlander and a fully paid-up member of the human race’. But he never really got to grips with that dark cloud in his life.”
Mr Barrett’s successor, Mike Crockart, who also lost his seat in the election, said: “We have lost one of our greatest, whose appeal spread far wider than all our politics. Charles showed wit, charm and intelligence every time he rose to speak.”
Margaret Smith, former MSP for Edinburgh West, said: “He was a unique character. He had it all – fantastic communication skills, because it came from the heart, a passion for people, a terrific wit and a real intelligence.
She said Mr Kennedy had suffered the sadness of his father’s death in April and then losing his seat in the election.
“Some of us just assumed Charles would bounce back. We’ll never know now what he might have been able to do.”
Mike Pringle, former Edinburgh South MSP, said Mr Kennedy was the best leader he had known and claimed the party would not be in its current “dire mess” if he had remained at the helm.
He said: “If he had been in charge in 2010 we would not have gone into coalition with the Tories, and we would not have reneged on student fees.”