16-dione is a soluble precursor of pentacene, which can be converted into pentacene by irradiation in the solid-state. Its photoconversion process in spin-coated films was monitored by UV-visible absorption and IR spectroscopy. A small amount of high-boiling-point additives in the chloroform spin-coating solution promoted photoconversion to obtain high quality films suitable for FETs. The FET mobilities showed a correlation with the additives' boiling points and dissolution abilities, indicating that the retainment of a semidry state during photoconversion is essential to the complete photoconversion. Photoirradiation conditions (irradiation intensity, duration and substrate temperature) were optimized to achieve a field-effect mobility of 0.86 cm 2 V À1 s À1 , comparable to the performance of vacuum-deposited pentacene films. The prepared films have a partially crystalline morphology different from that of vacuum-deposited films. The high FET mobility of the photoconverted film is attributed to continuously connected grain boundaries arising from partial crystallinity.