An empirical relationship between carbon and hydrogen contents of coals and their Measured vitrinite reflectances R-v, (max) (henceforth R-o) determined from published data for 210 samples of Australian coals from more than 60 different seams, shows that R-o depends upon both the carbon and hydrogen content via the equation %R-o= -5.035 + 0.0804%C - 0.184%H. This equation allows prediction of the change in R-o when the carbon and/or hydrogen content vary. The equi-reflectance lines plotted on a Seyler chart using thin equation allow prediction of the variable suppression of R-o caused by changes in hydrogen content. (c) 2006 Elsevier B.V. All rights reserved.