Four-wave mixing (FWM) with two-photon absorption (TPA) resonances is shown to allow time-resolved studies of ultrafast processes involved in photochromism in three-dimensional spiropyran/PMMA samples on the femtosecond time scale. An FWM signal was produced using 130-fs pulses of 800-nm Ti:sapphire-laser radiation as a TPA-resonant pump and frequency-tunable pulses of an optical parametric amplifier as a time-delayed probe. Two-photon resonances allow strong one-photon absorption to be avoided, giving an access to any point inside a 3D photochromic sample. An ultrafast dynamics of photochromic transformations is revealed by comparing the time-resolved FWM signal with the cross-correlation of pump and probe pulses. (C) 2003 Elsevier B.V. All rights reserved.