Abstract. Let K, M ∈ N with K < M, and define a square K × K Vandermonde matrix A = A τ, − → n ¡ with nodes on the unit circle: Ap,q = exp (−j2πpnqτ /K) ; p, q = 0, 1, ..., K − 1, where nq ∈ {0, 1, ..., M − 1} and n 0 < n 1 < .... < n K−1 . Such matrices arise in some types of interpolation problems. In this paper, necessary and sufficient conditions are presented on the vector − → n so that a value of τ ∈ R can be found to achieve perfect conditioning of A. A simple test to check the condition is derived and the corresponding value of τ is found.