The electric field that drives separation and retention in electrical field flow fractionation (ElFFF) and cyclical electrical field-flow fractionation (CyElFFF) is a complex function of many parameters such as carrier ionic strength and pH, voltage, channel dimensions, flowrate, and electrode material. Currently there is no accurate or in situ method to measure the field during system operation. This paper introduces a technique to measure the effective electric field during ElFFF and CyElFFF operation using transient electrical spikes. With this technique we can determine the relationship between changes in carrier conductivity and flowrate during a run and their combined effect on effective field and retention in ElFFF. This technique can also be used to measure the voltage drop due to double layer capacitance in CyElFFF and the variation in effective field with frequency of the applied field. The measured effective fields for the CyElFFF and DC ElFFFtechniques are also tested with a high ionic-strength buffer solution as carrier. For a high ionic-strength buffer, DC ElFFF generates a near-zero effective field (0.2% in 100 s), whereas CyElFFFcan sustain much higher effective fields (similar to 8%) even at relatively high voltages. The ability to measure the effective field allows for experiments to provide better data and for tuning and optimization of the separation run.