Between 1806 and 1808 she made two complete voyages as a slave ship in the triangular trade in enslaved people.

On the second, in early 1808 as Thetis was coming to Barbados from Africa, she again drove off a French privateer in a single ship action.

After war with France resumed in 1803, Captain John Charnley acquired another letter of marque on 26 September 1803.

In late 1804 Thetis arrived at Barbados after having repelled the attack of a French privateer of 20 guns.

[6] On her way to Barbados, Thetis fell in with the French privateer Revanche, of 14 guns and 180 men, at 12°N 52°W / 12°N 52°W / 12; -52.

Their owners had rushed them into a last, legal journey, with the result that they had arrived in a poor state and under-provisioned.

[6] On the voyages to and from Brazil, Thetis on occasion stopped at Madeira, the Canaries, or Havana.