The electrophoretic mobility of colloidal alkane and silica particles has been measured over the temperature range from 10 to 60-degrees-C in both water and 10(-3) M KCl. Distinctly different temperature dependence is observed for the two types of particles. The nature of the effective charge at the interface is interpreted as deriving from different mechanisms for the two different surfaces. The increasing surface charge density with temperature for the alkane particles is consistent with an entropic mechanism.