To our knowledge, today the differential entropy inequality appears to be always ignored in simulating chemically reacting flows. In addition to satisfying the differential mass, momentum, and energy balances, simulations of chemically/biochemically reacting systems must satisfy the entropy inequality (the second law of thermodynamics). Common types of material behavior (Newton's law of viscosity, Fourier's law, Fick's first law) satisfy the differential entropy inequality automatically, but common empirical descriptions of chemical/biochemical reactions do not. The issue is illustrated with three examples including a laminar methane flame for which data are available from the literature. (C) 2011 Elsevier Ltd. All rights reserved.