Combining an effective Hamiltonian model of electronic structure with nonadiabatic molecular dynamics simulations, we calculate the recently measured transient photoelectron spectrum of I-2(-) dissociated inside a cluster of argon atoms. We find good agreement between calculated and experimental spectra. The transient spectral shifts reflect the dynamics of both the I-2(-) and argon degrees of freedom, revealing pathways and time scales for dissociation, recombination, and vibrational relaxation.