This report documents development of a computer-based equation of state (EOS) that covers the range of conditions from the air saturation line to beyond lunar reentry conditions. Efforts to ascertain the limits of accuracy of the EOS are also documented. The EOS is largely a computer-based version of the venerable AEDC Mollier Diagram for equilibrium air, c. 1967. The basis of this EOS is different from its predecessor, but the range of applicability is comparable and the purpose is the same. Efforts have been made to determine the accuracy of the EOS. Transport properties as well as thermodynamic properties are calculated. The envelope of the EOS will be described in detail in the report, but in general terms it is as follows. The minimum temperature is 27 K or the saturation temperature, whichever is greater for densities less than the critical density. The minimum temperature is 100 K for densities greater than the critical density. The maximum temperature is 5,000 K for densities greater than 100 kg/cu m and 20,000 K for densities less than 100 kg/cu m. The maximum density is 1,220 kg/cu m for temperatures less than 5000 K and 100 kg/cu m for temperatures greater than 100 kg/cu m. The minimum density is 10(exp -12) kg/cu m for temperatures less than 350 K and 10(exp -8) kg/cu m for temperatures greater than 350 K. Some parts of the envelope are of little practical use for the current purposes as will be discussed in the report. The reference temperature for energy is 0 K, and the reference pressure for entropy is 101,325 Pa. The AEDC Mollier 2008 EOS has reached a state of development such that it should be used for calculations and predictions. There are no functionally important restrictions for its use at AEDC. The EOS is a work in progress. This report is an interim report and is expected to be superseded by another report when significant changes are made.