Three days of negotiations between Labour and the Liberal Democrats to try and form a home rule administration were clearly taking a toll.
Just along the horseshoe of desks, his putative coalition partner, Jim Wallace, was still smiling. Perhaps it is the Irn-Bru, of which the Liberal Democrat leader is so fond - but something is certainly giving him extra fizz through the long days and nights of talking, though cynics say it is excitement at the prospect of a ministerial car.
Seven-strong teams from both parties, aided by civil servants, have been trying to marry two manifestos making sometimes conflicting promises. The scene for the marathon talks has been the Parliament's makeshift administration block on Edinburgh's George IV Bridge, close by the Royal Mile. The dismal 1960s edifice is the former home of Lothian regional council, tarted up inside with creamy paint and new office furniture.
Mr Dewar and Mr Wallace held a preliminary three hours of talks in the grandly-named "ministerial suite" on the fifth floor on Sunday, and have spent most of their waking hours there since. Unmemorable lunches and sandwiches have been ferried up from the old staff canteen. Mr Dewar's nickname of "The Gannet" is well merited and it is hardly likely that the vegetarian satay which so impressed one Liberal Democrat was adequate for Mr Dewar's formidable appetite.
In rooms along the fifth floor, civil servants have prepared policy drafts and groups from the Labour and Lib Dem camps have thrashed out common positions. Mr Dewar's team includes two fellow ministers, Henry McLeish and Sam Galbraith, and is backed by his Scottish Office special adviser, Murray Edler, and seconded Lottery administrator, John Rafferty, who is tipped to become his chief of staff.
What the Liberal Democrats lack in heavyweight experience they have made up in their enthusiasm for the action. Labour negotiators have grumbled that each time a deal seemed near the Lib Dems have raised another issue.
In turn, Mr Wallace's team have been angered by Labour spin doctors suggesting to reporters that the Lib Dems were about to cave in over student tuition fees and agree to a review rather than abolition. "There has been a concerted attempt to bounce us," said one source.
Face-to-face talks between the two leaders have taken place in an unprepossessing room across a rectangle of pushed-together desks. Both men have looked like the pensive solicitors they are. They have hardly been the "white- knuckle encounters" suggested by some, though Mr Dewar has become increasingly irritated by the Lib Dems "hard-to-get" tactics.
A cluster of reporters and camera crews have waited on the pavement outside, or taken refuge in Deacon Brodie's Tavern, until late into the night. Brief appearances by negotiators and their advisers have added little to the sum of anybody's knowledge, and journalists have spun each other. Andy Myles, a former chief executive of the Scottish Lib Dems and one of Mr Wallace's team, has been a regular visitor to the pavement, but only to draw on another cigarette. By yesterday tea time a rather dishevelled Mr Myles reckoned he was on his 54th version of revealing nothing at all.
Item-by-item compromises are being made to try and construct a joint programme capable of enduring a four-year parliament. Take, for example, the controversial tolls on the privately-financed road bridge to the Isle of Skye. The Lib Dems wanted to scrap tolls and Labour to retain them. The middle way is to keep tolls but give Skye and the mainland a package of support matching the burden of the pounds 5 charge.
Mr Dewar also offered the Lib Dems a review of PR for council elections, though he may be wondering whether the wearying events of the last few days may not be the finest advertisement for the system.