We report on the modulation characteristics of indium phosphide (InP) based microdisks heterogeneously integrated on a silicon–on–insulator (SOI) waveguide. We present static extinction ratios and dynamic operation up to 10 Gb/s. Operation with a bit–error rate below 1 × 10-9 is demonstrated at 2.5, 5.0 and 10.0 Gb/s and the performance is compared with that of a commercial modulator. Power penalties are analyzed with respect to the pattern length. The power consumption is calculated and compared with state–of–the–art integrated modulator concepts. We demonstrate that InP microdisk modulators combine low–power and low–voltage operation with low footprint and high–speed. Moreover, the devices can be fabricated using the same technology as for lasers, detectors and wavelength converters, making them very attractive for co–integration.