A measurement system, based on high performance GPS receivers and general purpose acquisition boards, for the evaluation of the synchronized harmonic phasors in the nodes of an electric distribution network is presented. In order to meet the requirements of different fields of application, two measurement procedures have been implemented: one is based on a fixed observation window, whereas in the other one the observation interval is a function of the actual power system frequency. Experimental tests are presented to characterize the measurement system: the results show that the proposed procedures allow the accuracy usually required in control and management of distribution systems to be reached.