Electrochemical Machining (ECM) is a non-conventional, non-contact technique, popular in the aerospace and automotive industries, used to machine ultra hard metal alloys. This work details a novel system, using ultrasound to dynamically measure the interelectrode gap. For greater accuracy, thermocouples have also been incorporated into the system to allow for the often significant temperature variation due to resistive heating. This gives time-resolved data for the discussion valency and overpotential during ECM. The time averaged dissolution valencies found using this technique are the same, to within experimental error, as those found in previous literature demonstrating the accuracy of this approach. These parameters determine the ECM process and are used in computer modelling; generally these have been assumed to be time invariant however, significant variation is found in this work. This variation has also been used to give insight into high current dissolution processes. Iron machined in sodium chloride (NaCl) electrolyte was initially studied to eliminate any possible effects of alloy composition which might be seen when machining more complex materials. More pertinent to industrial ECM, the machining parameters of the hard metal alloys Inconel 718 (In718) and Titanium 6/4 (Ti6/4) in both NaCl and NaNO₃ were also investigated. The current-voltage characteristics were found to have some similarities to those of iron and stainless steels. Similar results were also found for In718 and Ti6/4 using an ECM cell of a longer electrolyte flow path length.