We have developed a real-time phase-shifting interferometer capable of imaging of interfacial morphology with a depth resolution of similar to 25 Angstrom and lateral resolution of similar to 0.5 mum across a field of view of 2 x 2 mm(2). The method employs a phase-shifting algorithm employing five interferograms and image processing that yield a three dimensional digital representation of the surface relief. The time resolution of imaging is 0.1 s. This method is applied in situ to the (1 0 1) face of potassium dihydrogen phosphate crystals growing from an aqueous solution. We image the formation and evolution of solution-flow induced step bunches and determine their characteristic wavelength to be lambda(c) = 45 mum. This wavelength is within the range predicted by a stability theory based on balance between the diffusion interaction between steps and capillarity. The found value suggests that step-step interactions are the likely major factor for instability. (C) 2002 Published by Elsevier Science B.V.