In this work, we present values for the Mark-Houwink-Sakurada (MHS) and Stockmayer-Fixman (SF) constants for a series of homopolymers of para-substituted styrenes (4-X-styrene; X = OCH3, OCH2CH3, CH3, F, Cl, and Br) in THF at room temperature. The respective values of K (in 10(-5) dL/g) and alpha were: 0.685 and 13.2; 0.662 and 14.1; 0.740 and 8.41; 0.781 and 5.24; 0.726 and 8.95; 0.700 and 7.79. The respective values for K-theta (in 10(-4) dL/g) and K' (in 10(-7) dL/g) were: 6.01 and 16.1; 6.22 and 9.07; 7.64 and 17.4; 5.59 and 23.7; 6.29 and 17.3; 4.44 and 10.3. These constants were measured using size-exclusion chromatography with on-line viscometry. As part of this work, we investigate the applicability of common model fitting procedures to this method of measuring MHS/SF constants and the effect of uncertainties in their estimated values on the accuracy of molecular weight analysis.