This paper presents definitive values of the zero-pressure thermodynamic properties of the stable naturally occurring isotopes and the normal isotopic mixtures of the following monatomic gases: Hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), fluorine (F), sulphur (S), chlorine (Cl), argon (A), bromine (Br), iodine (I), mercury (Hg). These values cover the range 100 to 5000 deg R at the intervals indicated by the numbers in parentheses: 100(5)130(10)240(20)500(50)1000(100)2000(200)5000. They have been computed by the methods of quantum statistical mechanics from present best values of the relevant (a) general physical constants, (b) isotopic weights and fractional abundances, (c) spectroscopic data. Computational accuracy well beyond present physical accuracy has been maintained for practical reasons, final results being given to seven significant figures. Present physical accuracy is indicated by charts of estimated uncertainties. For some of the monatomic gases here considered, accurate values of their zero-pressure thermodynamic properties hitherto have been unavailable; for others the data presented are the first based upon recent spectroscopic and other input data to be made available at temperatures sufficiently close for convenient practical application.