Two-wavelength phase-shifting interferometry is applied to an interference phase-measuring microscope enabling the measurement of step heights. The surface is effectively tested at a synthesized equivalent wavelength λeq = λaλb/|λa - λb| using measurements made at wavelengths λa and λb. The rms repeatabilty of the technique is λ/1000 at the equivalent wavelength. To improve the precision of the data, the phase ambiguities in the single wavelength data are removed using the equivalent wavelength results to determine fringe orders. When this correction is made, a measurement dynamic range (feature height/rms repeatability) of 104 is obtainable. This technique enables features of up to 10 μm in height to be measured with a 10x objective.