Excimer formation and dissociation in 9,10-dichloroanthracene is investigated in solution phase (low/high concentration) and monocrystalline state (α/β structure) at room temperature by ultrafast absorption spectroscopy combined with streak-camera fluorescence experiments. The transition from the monomeric to the excimeric state is accompanied by a red-shift of the fluorescence and a blue-shift of an excited state absorption band. Excimer formation in solution occurs in the 1 ns time regime. In the pre-arranged crystalline state of the β-form a strong acceleration of excimer formation to about 1 ps is observed. The slower formation time in solution is attributed to a diffusion process before the two molecules arrange in an ideal excimer geometry. The crystalline structure of the β-form does not allow to reach the optimal geometry, leading to significantly smaller spectral shifts than in solution. For the α-form spectral dynamics typical for the excimer formation are not observed.