We have studied the thermal conversion of 1,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline] (6-nitro-BIPS) in trifluoromethane (CHF 3 ) and nitrous oxide (N 2 O) from near-critical to liquid-like density regions. The rate constant decreases with increasing solvent density, which can be simply inferred from the density dependence of the dielectric constants of fluids. The activation energy, however, tends to decrease with increasing solvent density in the measured region in contrast to the dependence on the dielectric constant in liquid solutions. Desolvation due to the charge recombination and the conformational change may be the key to interpret this experimental result. We have also observed the aggregate formation and the deaggregation process of 6-nitro-BIPS in N 2 O.