Using a generalized Gibbs equation and a general form for the entropy flux, the nonlinear thermodynamics of micropolar fluids is studied. From Müller’s scheme, nonlinear constitutive equations are obtained. The classical law for the rotational viscosity is replaced by a new relation which leads to finite propagation of spin waves. The Gibbs equation and the entropy flux are in good agreement with the Enskog’s theory of dense gases through a Chapman-Enskog development. © Copyright by Walter de Gruyter & Co.