A model equation derived for the viscosity-average degree of polymerization (v) for the incremental addition of modifiers, v = K{(exp{arx1} − 1) + [2/(2 + exp {x1r})]a(exp{arx2} − exp{arx1})}1/a indicates a minimum should occur in a curve of v versus conversion at which the increments are added, x1. Incremental addition of commercial tert-dodecyl mercaptan in the laboratory to SBR recipes at 5°C. showed effects contrary to those predicted by the model equation, but incremental addition experiments with tert-nonyl mercaptans resulted in a minimum in viscosity versus x1 curves, changed the molecular weight distribution, and required 34 wt.-% less mercaptan than did the tert-dodecyl mercaptan control (all the tert-dodecyl mercaptan added initially). Incremental addition of tert-nonylmercaptan in pilot plant 20-gal. autoclave reactors gave essentially the same results found in the laboratory bottle experiments. The stress-strain and mixing properties of the incrementally modified polymer and the tert-dodecyl mercaptanmodified control were essentially the same within experimental error. Conditions required for successful application of the incremental modifier technique are given.