Jamieson, Fausset and Brown on Psalms 106:28-30

28-30. sacrifices of the dead--that is, of lifeless idols, contrasted with "the living God" (Jr 10:3-10; compare Psa 115:4-7; 1Co 12:2). On the words,

joined themselves to Baal-peor--see Nu 25:2, 3, 5.

Baal-peor--that is, the possessor of Peor, the mountain on which Chemosh, the idol of Moab, was worshipped, and at the foot of which Israel at the time lay encamped (Nu 23:28). The name never occurs except in connection with that locality and that circumstance.