It is known that [Fe (2-mephen)3] (ClO₄)₂ in the solid state is an iron(II) spin-crossover system which shows light-induced excited spin state trapping (LIESST). The thermal spin-crossover behaviour of the complex [Fe(2-mephen)₃]²⁺ embedded in various polymer matrices is similar to the solid-state behaviour, and a light-induced long-lived excited state is observed at temperatures below 50 K. Relaxation curves show that polymer matrices are not very homogeneous media.