Sir Reg Empey, a UUP member of the Executive, said last night that, while it was always clear that Sinn Fein would be in power because of its electoral support, he was still unconvinced of the party's intentions.
He said: "It requires the remainder of this process to determine whether we get the twin objectives of decommissioning and devolution. In those circumstances, we will be able to come to a more comprehensive judgment, but the jury is still out.
"The only circumstance in which confidence in the community will begin to be established is when decommissioning happens. When that does, I think we will be able to turn over a new page in our history, but I think until that happens there will still be very grave doubts."
Sir Reg criticised the DUP. While still opposing the institutions set up under the Belfast Agreement, the party was "riding into office on the backs of the Ulster Unionists", he said.
He accused the DUP of putting up only a token resistance in the Assembly chamber yesterday. "They are in government with Sinn Fein and they have to admit that, both to themselves and to their supporters."
The most unexpected of yesterday's appointments to the Executive was the naming of the UUP's Mr Sam Foster as Minister for the Environment. Mr Foster will take control of an office previously dubbed the "Department of Everything". He will assume responsibility for planning control, environmental protection, local government, road safety, transport, driving tests and vehicle testing and licensing.
Mr Foster promised last night to develop a long-term approach to waste management. "My new role will give me the opportunity to promote a better environment for everyone", he said.
The SDLP leader, Mr John Hume, said that the successful nomination of ministers represented a historic development. Standing beside the Deputy First Minister, Mr Seamus Mallon, Mr Hume said: "The Executive is now created and it is a huge and historic development. This is the first time in our history that representatives of all sections of our people will be working together in government. It will transform our society and replace the politics and violence of the past."
Mr Mallon said: "We have to ensure that this Executive works so well and so effectively and becomes so much a central part of the lives of the people here in Northern Ireland that it would become unthinkable that, in a month's time or two months' time, or no matter how many months' time, the political process and the people of Northern Ireland would not have it as a focal point."
He added: "Decommissioning has to happen, just as devolution had to happen."