Transient absorption spectroscopy was used to investigate the dynamics of the photochromic indolinobenzospiropyran reaction in toluene solution and in phosphatidylcholine bilayers (1,2-dimyristoyl-sn-glycero-3phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dioleoyl-sn-glycero3-phosphocholine (DOPC)). After excitation with UV light, colorless (R/S)-2-(3',3'-dimethyl-6-nitro-3'Hspiro [chromene-2,2'-indol]-1'-yl) ethanol derivatives are converted to colored merocyanine products in high yield; Phi = 0.45 in DMPC liposomes. We find that the reaction occurs in the bilayer aliphatic region in the gel (P-beta') and liquid (L-alpha) phases. The Arrhenius activation energy for the isomerization in DMPC bilayers was similar to 3.5 times larger in the liquid phase (L-alpha, E-a, = 26.0 +/- 1.0 kJ mol(-1)) than that in the gel phase (P-beta', E-a, = 7.3 +/- 1.6 kJ mol(-1)). Analysis of the isomerization rate constant temperature dependence allows an estimation of the bilayer viscosity and free volume properties in the L-alpha phase.