They have been named locally as Marcos Cervera and Esteban Garcia and were staying at the four-star Mayflower Hotel, Spanish media reports.
Police were scrambled to the posh hotel yesterday at around 1.48pm after receiving a report Marcos had been found dead.
A spokesman added: “Officers attended with London Ambulance Service and London Fire Brigade colleagues also in attendance.
“A male was pronounced dead at the scene.
“Officers are working to inform next of kin. A post-mortem will be held in due course.
“At this early stage the death is being treated as unexplained.”
Esteban was rushed to to hospital where he remains in a critical condition.
Around 29 guests were evacuated from the hotel on Trebovir Road and taken elsewhere to stay while emergency crews investigate.
Scotland Yard added: “An investigation is being undertaken by Kensington & Chelsea CID in conjunction with the Health and Safety Executive.
“Initial tests indicated a high reading of carbon monoxide, though the investigation has not yet concluded.
“The hotel was evacuated and remains closed while the investigation continues.
“A cordon is in place and the scene has been made safe.”