We have evaluated the structure of high-reflectivity InP/InGaAsP distributed Bragg reflectors grown by chemical-beam epitaxy at a growth rate of 3 mu m/h. Transmission electron microscopy combined with electron energy loss spectroscopy indicate lateral fluctuations in the GaInAsP alloy composition, clearly visible in the 490-510 degrees C growth temperature range, but less perceptible between 460 and 480 degrees C. In the higher temperature range, the modulation in composition shows a higher frequency along the [1 (1) over bar 0] direction than along [1 1 0].