Chemical reactions are manifestations of the dynamics of molecular valence electrons and their couplings to atomic motions. Emerging methods in attosecond science can probe purely electronic dynamics in atomic and molecular systems [1][2][3][4][5][6] . By contrast, time-resolved structural-dynamics methods such as electron 7-10 or X-ray diffraction 11 and X-ray absorption 12 yield complementary information about the atomic motions. Time-resolved methods that are directly sensitive to both valence-electron dynamics and atomic motions include photoelectron spectroscopy 13-15 and high-harmonic generation 16,17 : in both cases, this sensitivity derives from the ionization-matrix element 18,19 . Here we demonstrate a time-resolved molecularframe photoelectron-angular-distribution (TRMFPAD) method for imaging the purely valence-electron dynamics during a chemical reaction. Specifically, the TRMFPADs measured during the non-adiabatic photodissociation of carbon disulphide demonstrate how the purely electronic rearrangements of the valence electrons can be projected from inherently coupled electronic-vibrational dynamics. Combined with ongoing efforts in molecular frame alignment 20 and orientation 21,22 , TRMFPADs offer the promise of directly imaging valenceelectron dynamics during molecular processes without involving the use of strong, highly perturbing laser fields 23 . Figure 1 provides a conceptual overview of our method. Carbon disulphide, CS 2 , is a molecule that exhibits all the features generic to polyatomic dynamics: vibrational mode coupling, conical intersections, spin conversion and photodissociation. As such, its non-adiabatic photodissociation reaction CS 2 (X ) Fig. 1, provides an excellent test. In this work we combine experimental measurements with theory to demonstrate how the TRMFPAD images the evolution of the valence-electronic structure of an excited-state wavepacket during the complex, coupled electronnuclear processes inherent to chemical reactions. TRMFPADs probe both nuclear and electronic degrees of freedom through the photoionization matrix elements, d(t ) = + ; e |μ ·E| i (t ) . These matrix elements describe how the initial wavepacket, and its subsequent evolution in time ( i (t )), is projected onto the ionization continuum, consisting of the cation ( + ) and photoelectron ( e ) states, through dipole coupling (μ) with the laser field (E; refs 18,24). In the case of polyatomic molecules, i (t ) and + are composed of coupled electronic and vibrational components (see Supplementary Information). The significance of the TRMFPAD can be understood by considering the intimate relationship between e and the electronic part of i (t ). For example, under the well-known Born (plane-wave) approximation, 1 Steacie Institute for Molecular Sciences, National Research Council, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada, 2 Department of Chemistry, University of Copenhagen, Copenhagen DK-2100, Denmark, 3 Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada. *e-mail: alb...