The ability of four adsorption isotherm equations to predict the service life of organic vapor purifying cartridges was examined. Breakthrough curves (plots of penetration fraction versus time) were determined for ethanol (64175), carbon-tetrachloride (56235), acetone (67641), chloroform (67663), and n-hexane(110543) (hexane) in three lots of Wilson, two lots of Pulmosan, and one lot of Norton organic vapor cartridges. Breakthrough times for 1% penetration were determined from the curves. These values were used to obtain values of cartridge capacity for each organic vapor concentration tested. Plots of capacity versus concentration were constructed. The experimental curves were compared with those predicted by the Freundlich, Langmuir, Dubinin/Radushkevich (D/R), and Hacskaylo/LeVan (H/L) isotherm equations. All four equations fit the data for chloroform, carbon-tetrachloride, hexane, and acetone equally well. The authors conclude that the Langmuir, D/R, and H/L equations are equally satisfactory for describing the experimental data. The Freundlich equation does not fit the ethanol data as well as the other three. When the four equations were considered from the standpoint of goodness of fit with the experimental data under specified conditions and having desirable characteristics such as including a temperature effect, ease of application to the data, whether they reduce to Henry's Law at low concentrations, and whether the parameters have physical meaning, no single equation can be said to be clearly better than the others. The choice of an equation for correlating organic vapor respirator cartridge breakthrough data depends on which equation characteristics are most important to the user.