Forced Rayleigh scattering(FRS) has been used to measure the self-diffusion and tracer diffusion of nearly symmetric, well-entangled, poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) block copolymers. A procedure has been developed for the labeling of these polymers with a photochromic molecule (4’-(N,N-dimethylamino)-2-nitrostilbene-4-carboxylic acid), an essential requirement of the FRS technique. The temperature and molecular weight dependence of diffusion in PEP-PEE 1, a completely disordered block copolymer, was studied using three tracers : PEP-PEE 1, PEP-PEE 2, and PEP-PEE 3 (M(n) = 3.1(5) x 10(4), 5.0(1) x 10(4), and 8.1(2) X 10(4), respectively). The data clearly indicate that, in this region of the phase diagram, block copolymers behave no differently than linear homopolymers, i.e., they appear to obey reptation dynamics. These three tracers were also studied in a matrix, PEP-PEE 2, which exhibits an order-disorder transition (ODT) at 96-degrees-C. In the ordered matrix, the mobility of all three tracers was suppressed relative to the disordered state. The temperature dependence of the diffusivity was smooth through the ODT. For the two higher molecular weight tracers, fluctuations retard chain mobility at temperatures well above the ODT, which explains the observed insensitivity of diffusion measurements to the ODT itself.