This paper deals with the evaluation of the critical-state parameters with respect to the matric suction for saturated and unsaturated undisturbed residual clayey soils from Turkey. In order to conduct the unsaturated triaxial compression testing procedures a conventional triaxial compression apparatus was redesigned. The data for critical-state conditions from these tests are presented with respect to matric suction, based on the critical-state parameters of M, q(0), Gamma, lambda, which is commonly proposed by many authors. The critical state of the unsaturated samples is compared with that of the saturated samples. This experimental study has demonstrated that matric suction has no influence on parameters of M and lambda. The parameters of M and lambda are approximately 0.85 and 0.074 respectively for saturated and unsaturated conditions. The relationships between matric suction (u(a)-u(w)) and the intercepts q(0) and Gamma have been observed as nonlinear, and thus they can be defined as a function of matric suction (u(a)-u(w)). Furthermore, a method is developed to predict the intercepts q(0) according to matric suction for unsaturated clayey soils. (c) 2007 Elsevier B.V. All rights reserved.