The linewidths of exciton features in spectra from semiconductor quant
um wells normally correspond directly to the width of the probability
distribution of the disorder potential causing the broadening. This is
because the mass of the exciton is large, so its motion in the potent
ial is essentially classical. We show that in a microcavity, where pol
ariton mixing leads to much smaller masses, quantum mechanical effects
are expected to cause significant spatial averaging over the disorder
potential and hence motional narrowing of the spectral Lines. Our pre
diction is verified by experimental measurements of linewidths near re
sonance in a high quality microcavity structure.