Abstract. We derive formulas for the backward error of an approximate eigenvalue of a * -palindromic matrix polynomial with respect to * -palindromic perturbations. Such formulas are also obtained for complex T -palindromic pencils and quadratic polynomials. When the T -palindromic polynomial is real, then we derive the backward error of a real number considered as an approximate eigenvalue of the matrix polynomial with respect to real T -palindromic perturbations. In all cases the corresponding minimal structure preserving perturbations are obtained as well. The results are illustrated by numerical experiments. These show that there is a significant difference between the backward errors with respect to structure preserving and arbitrary perturbations in many cases.Key words. palindromic matrix pencil, palindromic matrix polynomial, perturbation theory, eigenvalue backward error, structured eigenvalue backward error AMS subject classifications. 15A22, 15A18, 47A56, 15A60, 65F15, 65F30, 93C73 DOI. 10.1137/140973839 1. Introduction. Given n × n matrices A 0 , . . . , A m , the corresponding matrix polynomial P (z) :We consider such eigenvalue problems for the special case that the coefficient matrices of P (z) satisfy certain symmetries. This is indicated by stating that the ordered tuple (A 0 , . . . , A m ) belongs to S, where S ⊂ (C n×n ) m+1 . In particular, given λ ∈ C, we are interested in perturbations (Δ 0 , . . . , Δ m ) ∈ S to (A 0 , . . . , A m ) ∈ S that are minimal with respect to a specified norm such that λ is an eigenvalue of the perturbed polynomialThe norm of such a minimal structure preserving perturbation is called the structured backward error of λ as an approximate eigenvalue of P (z). We refer to this also as the structured eigenvalue backward error of λ with respect to P (z) and S. In contrast, we refer to the norm of a minimal but not necessarily structure preserving perturbation to P (z) such that λ ∈ C is an eigenvalue of the perturbed polynomial simply as the eigenvalue backward error of λ with respect to P (z).Matrix polynomials with symmetries in their coefficients are referred to as structured matrix polynomials. For example, the coefficients of Hermitian matrix polynomials are all Hermitian matrices, i.e., they satisfy, A