Scandinavian Airlines is to lease up to 17 Boeing 737-700/800 aircraft, and take additional Airbus A320s, as part of its effort to bring greater type uniformity to its fleet.
The carrier is phasing out its Boeing MD-80 and older-variant 737s - a total of 20 aircraft - at its Stockholm and Oslo hubs.
SAS Group says it has agreed to lease five 737-800s from US lessor GECAS. It has also signed a memorandum of understanding with GECAS covering the lease of 12 737-700s.
The company operates another 17 MD-80s at its Copenhagen hub, but says that these will similarly be withdrawn and replaced with A320s.
It says the replacement will start in 2012 and be completed by 2016.
SAS Group adds that the agreements mean there will be "limited aircraft investments" in terms of capital expenditure this year.
Chief executive Rickard Gustafson says that the decision will "reduce complexity".
SAS Group has long intended to replace the MD-80s but had been looking at the potential of the re-engined A320 as well as a possible re-engined 737, with a view to making a switch around 2015.
The company has also previously considered the Bombardier CSeries as an option for MD-80 replacement.
It already operates 67 newer-variant 737s and 12 A320s across its three capital bases.