Duhaime's Law Dictionary

Rule in Aerocide Definition:

A Canadian legal principle applicable in employment law which holds an employer to the reasons for a dismissal expressed at the time of dismissal, and prohibiting the attempt to later advance other reasons.

flag of CanadaSo named after the 1965 Ontario case in which it was originally expounded, USWA v Aerocide Dispensers Ltd., in these words:

"The board is justified in a case of challenged discharge to hold the employer fairly strictly to the grounds upon which it has chosen to act against an employee who consequently feels himself aggrieved. This is not to say that the board should be overly technical in assessing an assigned cause of discharge but it does mean that it ought not to permit an assigned cause to be reformed into one different from it merely because the evidence does not support the assigned cause but rather one something like it."

In Re York, the labour arbitration tribunal articulated the rule as follows:

"[A]n employer is limited to justifying its action in dismissing an employee to the grounds it put forward at the time of discharge."


  • Re City of York Board of Education and CUPE 37 LAC (4th) 257 (Ontario, 1993)
  • USWA v Aerocide Dispensers Ltd. 15 LAC 416 (1965)

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