Environment Canada issued heat warnings on Monday for Ottawa, most of eastern Ontario and Gatineau.
The weather agency said Monday afternoon's forecast high in Ottawa is around 31 C, with humidity making it feel more like 40.
It's unlikely to drop below 20 C at night with overnight humidity making it feel like it's 25. The temperature is expected to be the same on Tuesday.
Renfrew County and the rest of western Quebec don't have any Environment Canada weather alerts, while the Belleville area has a special weather statement about humidity that may make it feel close to 40.
There is some relief in the forecast with Wednesday's high expected to be about 25 C with a 70 per cent chance of showers.