In February CAS overturned the bans and sanctions imposed on 28 out of 39 Russian athletes for alleged anti-doping rule violations.
Cross country skier Legkov was cleared of any anti-doping rule violation while bobsledder Zubkov's appeal was partially upheld by CAS which did, however, say that the latter was responsible for an anti-doping rule violation.
CAS said its investigation found that the evidence put forward by the IOC "did not have the same weight in each individual case" regarding Legkov and Zubkov.
"We are unsatisfied with the decision and the motivation and we will be appealing," IOC spokesman Mark Adams told reporters on Thursday. He said the IOC would take the matter of the two athletes to the Swiss Federal Tribunal.
Adams said the IOC had yet to receive the full CAS decisions on the two athletes who were part of the group of 39 Russians who had appealed their initial Olympic lifetime bans and stripping of Olympic medals.
It is expected the IOC will lodge further appeals once CAS starts to publish detailed reasons for clearing the Russians athletes of any wrongdoing.
The Russians were banned from competing under their country's flag at the Pyeongchang Winter Games in February and took part as neutral athletes under the Olympic flag over the country's doping scandal.
Allegations of a widespread doping cover-up involving Russian athletes across many sports, and during the 2014 Sochi Winter Olympics, led to a string of investigations by the IOC and the World Anti-Doping Agency (WADA).
World athletics body IAAF has banned Russia's athletics federation since 2016 while the country's anti-doping agency has also been suspended.
Russian authorities have acknowledged transgressions but have repeatedly denied the existence of a systematic state-sponsored doping programme.
(Reporting by Karolos Grohmann, editing by Pritha Sarkar)
