Aberdeen's fleet of hydrogen buses has been taken off the road due to a "technical issue".
The buses were introduced in January last year as part of a project aimed at zero emissions and low noise travel.
First Bus said an issue had been identified with the 15 hydrogen buses and the vehicles had been taken off service until the technical issue could be "better understood".
Replacement buses are being brought in until it is resolved.
First Bus said: "To minimise the impact of this temporary vehicle withdrawal, we have reallocated low emission Euro VI buses from our Glasgow fleet.
"Consequently we don't anticipate any problems for customers planning a journey on our services."
Bus manufacturer Wrightbus said it expected most of the vehicles to be back in service over the coming days.
Ian Gillot, parts and services group director, said an issue with a mounting bracket at the rear of a bus was detected during a routine check on Thursday.
"After undertaking a thorough maintenance check, we have decided to upgrade and replace the part across the fleet to ensure we don't see a repeat of the issue," he said.
"This is a mechanical problem and not a hydrogen fuel-related issue," he added.
Mr Gillot said they had reassured First Bus that staff in Ballymena, Northern Ireland, were "rectifying the problem swiftly".