Let m(A) (0) denote the nullity of a given matrix A of order n. Set A(alpha) for the principal submatrix of A obtained after deleting the rows and columns indexed by the nonempty subset a of {1, . . . , n}. When m(A(alpha))(0) = m(A)(0) + |alpha|, we call alpha a P-set of A. In this paper, we classify all of the trees T for which there exists a matrix A whose graph is T and containing a P-set of maximum size. Our characterization does not depend on whether the acyclic matrices are singular or nonsingular. (C) 2013 Elsevier Inc. All rights reserved.