Fire crews were called to a house in Barcroft Street, Cleethorpes on Wednesday evening, after reports that a CO alarm was sounding.
Firefighters entered the property and found the body of a woman, in her 50s, in a ground-floor flat.
A man in the upstairs flat was rescued, Humberside fire service said.
Andy James, who was the officer in charge at the incident, said: “One of the occupiers had a working carbon monoxide detector that alerted her to presence of the gas.
“This gave enough early warning to call us out and we were able to evacuate most of the surrounding properties.
“Sadly it was too late for one of the occupiers but I have no doubt the situation could have been much worse had the alarm not sounded.”
The source of the carbon monoxide is being investigated.