This paper investigates the deviation in frictional characteristics from Stokes flow theoretical expectations for liquid flow through fused silica microtubes. Pressure drop data is used to characterize the friction factor for tube diameters ranging from 20−150 µm and over a Reynolds number range of 20−2000. Distilled water, hexane, and isopropanol were used in this study based on their distinct polarity and viscosity properties. Deviation from Stokes flow theory is observed in tubes with diameters less than 75 µm. The deviation appears to be independent of Re, represents a decrease in the friction factor and is similar for all three liquids at a fixed diameter. The measured deviation is 0% for 100−150 µm range and increases as the diameter of the tube decreases, from 5% at 75 µm to 30% at 20 µm. Possible sources for the observed deviation include shear heating, property variation with pressure, and surface phenomena. Of these effects, shear heating and property variation with pressure cannot explain the observed variations.