High-resolution electron holography in the transmission electron microscope has the capability to profile the spatial variation of the electrostatic potential in semiconductors, at subnanometer resolution. We have used electron holography to measure the internal electrostatic potential and fields across quantum wells with well thickness ranging from 2 to 10 nm, at a nominal indium concentration of x=0.13. A comparison of field strength versus well width shows a precipitous drop in field strength beyond 6 nm. A close look at the microstructure of the widest well shows additional contrast at the growth interface. An explanation consistent with both observations supports the possibility of compositional fluctuations in combination with strain relaxation at the early stages of growth of the quantum well. © 2004 American Institute of Physics.