Electroacoustics and laser Doppler electrophoresis were employed to measure the mobility of surface-modified silica colloids in ethanol as a function of the ionic strength. Sufficiently low volume fractions were chosen to exclude effects of interparticle interactions. At high ionic strength, the electrophoretic mobility mu(e) is equal to the (electroacoustic) dynamic mobility mu(d) at 3.3 MHz. However, the ratio mu(d)/mu(e) increases significantly to similar to 5 at low ionic strength. This increase may be related to the porous outer layer of the surface-modified silica spheres. (c) 2014 Elsevier Inc. All rights reserved.