The Canada Revenue Agency allows you to deduct legal fees incurred to increase support, to defend against a claim to decrease support, or to collect unpaid support. Fees to establish child support for the first time are also deductible. Fees for the divorce itself are not tax-deductible.
Revenu Québec has the same rules save that the payor of support (rather than just the recipient, as in the case for the Canada Revenue Agency) can also deduct legal fees relating to the establishment or modification of support.
So, keep your detailed invoices and receipts, to establish that the legal fees you paid were to establish, increase (or prevent a decrease of), or collect support. We provide a letter of attestation of the legal fees attributable to these deductible topics each year, at no additional charge.
Read this Federal Income Tax Department Interpretation Bulletin for more explanations: