The diffusion of fullerenes such as C60 and PCBM in organic semiconductors is a key factor in controlling the efficiency of organic solar cells, though it is challenging to measure and to control. A simple optical method based on photoluminescence quenching is developed to assess the diffusion of a quencher molecule such as C60 through a semiconducting polymer film, in this case made with the polymer polyfluorene. When the mobility of the polymer chains is reduced by chemical crosslinking, the diffusion coefficient of C60 can be reduced by up to three orders of magnitude. The diffusion of fullerenes such as C60 and PCBM in organic semiconductors can be measured using a novel simple optical method based on photoluminescence quenching. When the mobility of the polymer chains is reduced by chemical crosslinking, the diffusion coefficient of C60 can be reduced by up to three orders of magnitude.