Scanning transmission X-ray microscopy has been used to quanutatively map the composition of P3HT PCBM blend films in the vicinity of PCBM crystals formed during annealing at 140 degrees C The observed PCBM concentration profiles around these crytals have been fitted to Fick's second law of diffusion and the dirfusion constant found to be 2 5 x 10(-14) m(2) s(-1) The PCBM concentration at the crystal boundary was found to be 19% (v/v) and is interpreted, together with the annealing temperature of 140 degrees C, as a point on the biomodal line of the composition-temperature phase diagram P3HT is observed to be bulk-dominated, in contrast to the surface/interface-dommated diffusion observed in MDMO-PPV:PCBM blend films by Yang et al 1,2