We consider the stationary Stokes problem for a class of power-law fluids and prove functional type a posteriori error estimates for the difference of the exact solution and any admissible function from the energy class. We also discuss such error estimates for Bingham-type fluids. The main purpose of such an inequality is to give a directly computable measure of the difference between the exact and an approximate solution. The advantage of functional type a posteriori estimates is that they do not require special properties of the approximate solutions (e.g. Galerkin othogonality) or of the respective finite dimensional spaces used. (c) 2006 Elsevier B.V. All rights reserved.