A description of the importance of rate of change of frequency (ROCOF) measurements to the operation of electricity networks is given. The susceptibility of ROCOF measurements to common power system disturbances such as phase steps is described. A measurement campaign to observe ROCOF at multiple locations in an island grid dominated by renewable generation is described and some results are given. These captured ROCOF events are dominated by those associated with phase steps, which occur without significant change to underlying power system frequency. It is concluded that they constitute a “false” ROCOF event. A new algorithm is presented that attempts to remove the influence of the phase step and reduce the associated ROCOF error such that the reliability of ROCOF measurements can be improved in the presence of phase steps. The algorithm is then applied to some recorded waveform sequences from the island that contain phase steps and the results are presented. In one example, it is shown that a false ROCOF spike in excess of 100 Hz/s was reduced to less than 5 Hz/s.