In order to extend the experimental database relevant for the astrophysical gamma-process towards the unexplored heavier mass region, the cross sections of the Eu-151(alpha, gamma)Tb-155 and Eu-151 (alpha, n)Tb-154 reactions have been measured at low energies between 12 and 17 MeV using the activation technique. The results are compared with the predictions of statistical model calculations and it is found that the calculations overestimate the cross sections by about a factor of 2. A sensitivity analysis shows that this discrepancy is caused by the inadequate description of the alpha+nucleus channel. A factor of 2 reduction of the reaction rate of Eu-151(alpha, gamma)Tb-155 in gamma-process network calculations with respect to theoretical rates using the optical potential by McFadden and Satchler (1966 Nucl. Phys. 84 177) is recommended.