Three huge pipes have been sunk into Lake Monoun to pump out carbon dioxide and disperse it, government officials said.
Researchers had reported that the level of gas saturation in the crater lake had reached dangerous levels.
In 1984, the gas seeped out of the lake, suffocating 40 people living nearby.
A similar venting operation began two years ago at another site - Lake Nyos, where a gas cloud killed more than 1,700 people in 1986.
Project
The lakes are part of a ridge of dormant flooded volcanoes that runs from the Atlantic along the border between Cameroon and Nigeria.
The carbon dioxide emissions are a natural phenomenon, and the substance is rarely harmful in small doses.
However large quantities can build up on at the bottom of some lakes and may later be forced out as the result of a landslip or underground disturbance.
The three-year $400,000 project to ensure the lakes are made safe for local people is being funded by the US and Cameroon.
The head of the project, Gregory Tanyileke, was quoted by the Associated Press as saying the scheme at Lake Nyos had run into problems last month because of rusty pipes.
However it is hoped that the siphon will be operating again in about four months' time.