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This work reports the result of a comparative testing done on four phasor measurement units (PMU) from different manufacturers. Phase shift and attenuation at fundamental frequency, performance under system balanced and unbalanced conditions and performance under system variable frequency are the issues addressed for this comparative assessment. The results are presented in a comparative evaluation of features and performance of the PMU units. The results of this work reveal that data from the tested PMU units is of comparable accuracy only under nominal frequency operating conditions. At off-nominal frequency operation every tested PMU unit yielded a different phase and magnitude for the common measured voltage signal. The existing Synchrophasor standard does not specify phase or magnitude performance for off-nominal frequency operation. The standard is in the process of revision, and it is expected that the revised standard will specifically address performance requirements at off-nominal frequencies.