The paper deals with tracking problem for multiple-input multiple-output nonlinear systems under unmatched external disturbances. The existence conditions of the input–output block form (IOBF) with regard for the external disturbances are obtained. This form is the basis for the decentralized control design in a narrow formulation, without entering a dynamic compensator generating derivatives of control actions. Sliding mode observers used to estimate the unmeasured variables of the new coordinate basis and external influences. These observers are not required expansion of the state space due to dynamic models of the external disturbances and the reference signals.