Fast scanning chip calorimetry was used to perform an elaborate isothermal study of poly(3‐hexyl thiophene)/[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT/PC61BM) submicron layers. The used scanning rates of 30 000 K s−1 allow for a ‘true’ isothermal study, where non‐isothermal effects are avoided. Results were obtained over a wide temperature range for the 1:1 composition, used in organic solar cells, and for selected temperatures for the 3:7 composition. The results can be clearly interpreted according to the eutectic behaviour expected for this system, with the 1:1 composition being enriched in P3HT and the 3:7 composition being enriched in PC61BM. In both cases, the start of the melting trajectory corresponding to the eutectic transition coincided with the Tg of PC61BM. This is in agreement with earlier studies that report a vitrification effect caused by PC61BM. A bell‐shaped curve of isothermal crystallization rates could be constructed for the 1:1 composition, where the Tg of the mixed amorphous phase as well as of the separate components can be seen to play a role. For the 3:7 composition, clear indications are observed that a vitrified PC61BM phase can be formed in which isothermal crystallization takes place.