The fundamental derivative of gas dynamics is a purely thermodynamic property associated with the analysis of detonation processes and shock waves. A fluid with negative value of this derivative, called a BZT (Bethe, Zel'dovich, and Thompson) fluid, would present rarefaction shock waves. Also, close to conditions in which in the fundamental derivative of gas dynamics is equal to zero, entropy losses are small, potentially leading to improved efficiency in turbomachinery. The experimental evidence that BZT fluids exist is disputed, but published calculations based on equations of state (EOS) predict their existence. Here, calculations with more than 1800 pure substances using the original Peng-Robinson EOS, in its modified form known as PR78, and the Patel-Teja-Valderrama EOS have initially identified 185 organic substances with negative or near-zero, positive minimum value of the fundamental derivative of gas dynamics. The effect of uncertainties in critical properties and acentric factors on the predicted fundamental derivative of gas dynamics has also been evaluated. (C) 2012 Elsevier B.V. All rights reserved.