The purpose of this paper is to investigate when a weight ϕ on a partial O * -algebra M is a trace weighted by a positive self-adjoint operator Ω, that is, ϕ(X † 2 X) = tr(X † * Ω) * (X † * Ω) whenever X ∈ M s.t. X † ∈ L(X) and ϕ(X † 2 X) < ∞. It is shown that if M contains the inverse N of a positive compact operator such that the weak multiplication N 2 N is defined, then every weight ϕ on M + satisfying ϕ(N 2 N) < ∞ is a trace weighted by some positive trace operator.