The purpose of this paper is to investigate spectral properties of the transition operator associated to a multivariate vector refinement equation and their applications to the study of smoothness of the corresponding refinable vector of functions.Let, where M is an expansive integer matrix. We assume that M is isotropic, i.e., M is similar to a diagonal matrix diag(σ 1 , . . . , σ s ) withThe smoothness of Φ is measured by the critical exponentwhereWe assume that the mask a is finitely supported, i.e., the set suppa := {α ∈ Z Z s : a(α) = 0} is finite. Note that each a(α) is an r × r complex matrix. Let A := α∈Z Z s a(α)/d, where d := | det M |. We assume that spec (A) (the spectrum of A) is {η 1 , η 2 . . . . , η r }, where η 1 = 1 and η j = 1 for j = 2, . . . , r., where ⊗ denotes the (right) Kronecker product. Suppose the highest degree of polynomials reproduced by Φ is k − 1. LetThe main result of this paper asserts that if Φ is stable, then λ(Φ) = − log d ρ k s/2, where. This result is obtained through an extensive use of linear algebra and matrix theory. Three examples are provided to illustrate the general theory. All these examples have background of practical applications.