The viscosity of a partially ionized thermal hydrogen plasma has been calculated by taking into account electronically excited states with their abnormal transport cross-sections. The results show a strong dependence of viscosity on the presence of electronically excited states especially at high pressure. The insensitivity of viscosity respect to the choice of cross-sections of excited states at atmospheric pressure is explained on the basis of a compensation between diagonal and off-diagonal elements in the viscosity equation. (C) 2003 Elsevier B.V. All rights reserved.