Monomers for which the necessary information was obtained for comparison included the following: styrene, α‐methylstyrene, cis and trans‐propenylbenzene, allylbenzene, m‐methylstyrene, p‐methylstyrene, o‐, m‐, and p‐chlorostyrene, 2,3‐, 2,4‐, 2,5‐, 2,6‐, 3,4‐, and 3,5‐dichlorostyrene, p‐ethylstyrene, and p‐tert‐butylstyrene. In both free radical and ionic copolymerizations involving similar monomers, plots of the logarithms of the relative reactivities of several substituted styrenes toward the styrene and methyl methacrylate radicals versus the differences existing between the exaltations of substituted styrenes and the exaltation of styrene were strikingly similar to those in which Hammett's sigma values for the styrene substituents were plotted against the relative reactivities of the substituted styrenes.