Am I responsible for WSIB premiums the seller never paid?

In a share purchase, generally yes — unpaid WSIB premiums are a debt tied to the corporation's account, and since you now own that corporation, the outstanding amount continues to sit with it regardless of when it accrued, unless your purchase agreement specifically addresses it through a price adjustment or indemnity.

In an asset purchase, this is more nuanced. You're generally a separate legal employer opening your own account, but whether outstanding amounts on the seller's account can affect you, your rating, or your ability to register cleanly can depend on the specific facts of the transaction rather than being something your purchase agreement alone controls, since this involves a government account and program with its own rules for how it treats a change in business ownership.

Because this sits partly outside standard contract drafting, confirm the seller's WSIB account status directly before closing, and where amounts are outstanding, address payment as part of closing rather than assuming an asset structure automatically leaves it entirely behind. A Treadstone business lawyer can help review account status and coordinate this with your closing.