The results are expected in the early hours of Friday morning.
The Scottish Tories are “cautiously optimistic” in the Aberdeen South by-election, the Press Association understands.
Party MSP Douglas Lumsden is running in the seat, in a bid to replace former SNP Westminster leader Stephen Flynn.
Senior SNP figures have also appeared pessimistic, suggesting a heavy tactical vote in early counting, which has coalesced around the Tory candidate.
Mr Flynn stood down last month after he was elected as an MSP, with Stephen Gethins doing the same.
Both were quickly shepherded into John Swinney’s Government.
The campaign for Aberdeen South became a battle over the future of oil and gas, with the Conservatives, Reform and the SNP vying to persuade those in the city – dubbed the energy capital of Europe – they support the industry the most.
Candidates reported the fate of the industry was one of the key issues coming up on the doorsteps during canvassing sessions.
Former MP Richard Thomson is running for the SNP, while the Tory candidate is MSP Douglas Lumsden and Reform’s is Jo Hart.
Other candidates in the by-election are: Jorg Shelton Eckstein of the Scottish Greens, Scottish Liberal Democrat Mel Sullivan, Labour’s Nurul Hoque Ali, and David Ballantine of the Alliance for Democracy and Freedom.
Meanwhile, the Arbroath and Broughty Ferry contest has been focused on the cost of living, according to those running in the seat.
Lawyer and party adviser Lara Bird is contesting the constituency for the SNP, against Reform’s Bill Reid and local Labour councillor Heather Doran.
Jack Cruickshanks of the Scottish Tories and Tanvir Ahmad of the Liberal Democrats are also running in the seat.
Polling stations opened at 7am and closed at 10pm, with ballots to be counted overnight.
