Similarity parameters are derived to correlate the selectivity of mixing controlled parallel reactions within plug-flow reactors with two types of static mixing elements for both aqueous and viscous solutions. For the case of laminar reactor feed, the Kolmogoroff time is shown to correlate selectivity in turbulent reactors provided the reactor Reynolds number Re < Re-C. The upper limit Re-C for the reactor Reynolds number is constrained by the ratio of reactant feed-to-total volumetric flow rate. Moreover for larger reactor Reynolds number Re > Re-C, the inertial time scale defined in terms of a reactor feed length correlates selectivity. Scaling laws are derived for both reactor diameter and length and suggestions are made for predicting selectivity with larger turbulent feed rates.