The modification of turbulence in the edge plasma of the DIII-D tokamak at the transition from the low to the high mode of confinement is investigated with a phase-contrast imaging diagnostic. The amplitude and radial correlation length of the turbulence in the confinement region decrease at the transition, whereas the decorrelation time increases. The transition model of Biglari, Diamond, and Teny [Phys. Fluids B 2 (1990) 1], based on turbulence decorrelation by E x B velocity shear, is quantitatively substantiated by measurements of the theoretical control parameter. Further quantitative predictions of the theory are tested for the first time. (C) 2000 Published by Elsevier Science B.V.