Abstract The T1(π, π*)→T3(π, π*) absorption of acridine has first been observed at about 985 nm in ethanol. Temperature dependences of the fluorescence lifetime and the triplet yield of acridine were investigated in poly(vinyl alcohol) film, and it was found that the intersystem crossing from the first excited singlet state S1(π, π*) to T1(π, π*) occurs through both temperature dependent and independent processes. The activation energy of the temperature dependent process corresponds substantially to the energy difference between T3(π, π*) and S1(π, π*). The intersystem crossing from S1(π, π*) to T2(n, π*) followed by the internal conversion to T1(π, π*) was assigned to the temperature independent process.