Dimerization of molecular ions with parent neutrals was observed for the series benzene, methylbenzene, p-dimethylbenzene (all at 296 K), and 1,2,4,5-tetramethylbenzene (at 230 and 247 K). From the pressure dependence of the observed rates the rate constants k3 for collision-stabilized association and k(ra), for radiative association were assigned. The results are analyzed along with the prior results for 1,3,5-trimethylbenzene. The radiative association rate constants are all relatively low at these temperatures, ranging from 1.5 x 10(-13) (toluene) to 7.1 x 10(-12) cm(3) s(-1) molecule(-1) (mesitylene). The radiative association rate for benzene ion was below the limits of observation. The results all agree within a factor of 2 with the semiquantitative predictions of the "standard hydrocarbon" model. With additional assumptions, separate rate constants for photon emission from the collision complex and redissociation of the collision complex were derived. The photon emission rates (10-30 s(-1)) increase modestly with increasing complex size and are slightly larger than expected for typical hydrocarbon ions. The redissociation rates are of the magnitude expected fi;om simple RRKM modeling, but correct modeling of the size dependence requires some modification of the simplest picture of substituent effects.