Abstract. We show that if R is a commutative ring and ðS; Þ a strictly totally ordered monoid, then the ring ½½R S; of generalized power series is Baer if and only if R is Baer.2000 Mathematics Subject Classification. 13F25, 16W60.A ring R is called Baer if the right annihilator of every nonempty subset of R is generated by an idempotent. Baer rings were studied in [1,2,3,5,6,7,11]. By [5, Theorem 3] the Baer condition is left-right symmetric. Semisimple artinian rings, domains and the rings of n  n upper triangular matrices over division rings are Baer, where n ¼ 1; 2; . . ..A ring R is called a right pp-ring if each principal right ideal of R is projective, or equivalently, if the right annihilator of each element of R is generated by an idempotent. Baer rings are clearly right pp-rings. It was proved in [9] that if R is a commutative ring and ðS; Þ a strictly totally ordered monoid then the ring ½½R S; of generalized power series is a pp-ring if and only if R is a pp-ring and every S-indexed subset C of the set BðRÞ of all idempotents of R has a least upper bound in BðRÞ. In this paper we show that if R is a commutative ring and ðS; Þ a strictly totally ordered monoid, then the ring ½½R S; of generalized power series is Baer if and only if R is Baer. All rings considered here are associative with identity. Any concept and notation not defined here can be found in [12,13,14,15]. Let ðS; Þ be an ordered set. Recall that ðS; Þ is artinian if every strictly decreasing sequence of elements of S is finite, and that ðS; Þ is narrow if every subset of pairwise order-incomparable elements of S is finite. Let S be a commutative monoid. Unless stated otherwise, the operation of S will be denoted additively, and the neutral element by 0. The following definition is due to P. Ribenboim. See [12,13,14,15].Let ðS; Þ be a strictly ordered monoid (that is, ðS; Þ is an ordered monoid satisfying the condition that, if s; s 0 ; t 2 S and s < s 0 , then s þ t < s 0 þ t), and R a commutative ring. Let A ¼ ½½R S; be the set of all maps f : SÀ!R such that suppð f Þ ¼ fs 2 Sj fðsÞ 6 ¼ 0g is artinian and narrow. With pointwise addition, A is an abelian additive group. For every s 2 S and f; g 2 A, let X s ð f; gÞ ¼ fðu; vÞ 2 S  Sj s ¼ u þ v; fðuÞ 6 ¼ 0; gðvÞ 6 ¼ 0g. It follows from [14, 1.16] that X s ð f; gÞ is finite. This fact allows us to define the operation of convolution