AbstractFilms of the fullerene derivatives [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) are patterned on silicon nitride membranes using photolithography to study, with X‐ray spectromicroscopy, the lateral, solid‐state diffusion of fullerene derivatives into conjugated polymer films. After patterning of the fullerene film, a film of conjugated polymer is laminated on top and the structure is annealed in order to study lateral intermixing and facilitate measurement of fullerene miscibility. Lateral intermixing of polymer and fullerene readily occurs for poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) and regiorandom poly(3‐hexylthiophene) (RRa‐P3HT). A 42 wt.% miscibility of PC61BM in PBTTT is measured, while miscibilities of 20 and 41 wt.% are measured for PC61BM and PC71BM, respectively, in RRa‐P3HT, thereby demonstrating a significant difference in the miscibilities of these two fullerene derivatives. For regioregular poly(3‐hexylthiophene) (RR‐P3HT), incomplete lateral intermixing of fullerene and RR‐P3HT is observed with PCBM crystallite formation competing with the lateral diffusion of PCBM molecules into the polymer film.