Is mediation in an Ontario will dispute confidential, and can what's said there be used at trial?

Yes, mediation sessions in Ontario estate disputes are confidential, and this is one of the main reasons people are willing to speak candidly and explore settlement options they might not raise in open court. Discussions during mediation, offers made to settle, and positions taken for the purpose of trying to resolve the dispute are generally protected by settlement privilege, meaning they cannot later be used as evidence against you at trial if the mediation doesn't result in a settlement.

This protection is not absolute. It generally covers genuine settlement communications, not independent facts or documents that were already discoverable outside the mediation and simply happen to also come up during it. A signed settlement agreement reached at mediation is also generally not confidential in the same way, since it becomes a binding agreement the parties may need to rely on or enforce later. Because the line between a protected settlement discussion and an unprotected fact can matter in practice, it's worth confirming with your lawyer, before the session, exactly what confidentiality does and doesn't cover, so you can participate openly without inadvertently giving up more than you intended.