An analytical expression is derived for the current-time transient for electrochemical machining (ECM) using a planar tool and workpiece configuration. This is obtained as a function of such parameters as the initial interelectrode gap, applied voltage, electrolytic conductivity, valency, density and feed rate. Good theoretical fits to experimental data are found for the alloys titanium 6/4 (Ti6/4) and Inconel 718 (In718) using both sodium chloride and sodium nitrate electrolytes, demonstrating the applicability of this theory. The values of the electrolytic molar conductivity obtained for chloride and nitrate are consistent with the expected conductivity obtained from molar conductivity measurements. The mean valency values obtained for Ti6/4 and In718 are 3.5 +/-0.2 and 3.0 +/- 0.2, respectively. The fraction of the applied voltage used to drive the electrochemical surface reactions, V-0, has also been obtained. The variation in V-0 between alloys when using the same electrolyte and also for each alloy when using different electrolytes is attributed to differences in the thermodynamics of the removal of the metal from the surface metal oxide. For In718 using chloride electrolyte, an increase in V-0 is observed at higher applied voltages, consistent with a change in the electrochemical dissolution reaction. Analysis of the variation of V-0 at low applied voltages throughout the current-time transient has enabled the current-voltage characteristics of these surfaces electrochemical reactions to be determined, indicating Tafel behaviour. These data show this analysis to be a powerful methodology for understanding and measuring ECM characteristics under realistic ECM conditions.