There is something called a stepped up basis at death. So for instance you had a stock you purchased for $100 in this joint account. It is worth $200 at the date of death of the decedent.
The Original basis is $50 each since it is divided in half (100/2)
The surviving spouse inherits the decedent's half at the value as of date of death. So the inherited basis is $100 (200 / 2).
The surviving spouse basis is now $150 ( 50 orig + 100 step up) .
Recompute the basis for all holdings in the account or see if the broker has done it already for you.
