Trapped ions of different masses can be separated in space within a quadrupole ion trap, making it possible to perform efficient mass-selective photodissociation on a mixture of ions. This method employs an axial quadrupolar dc pulse (1 mu s) to force all ions into coherent radial motion; ions of the same mass-to-charge ratio are either in phase or phase-shifted by exactly 180 degrees. After this activating pulse, the ions continue to oscillate at their secular frequency in a coherent fashion so that ions of the same mass-to-charge ratio simultaneously cross the z-axis (r = 0) twice per secular cycle. At this specific moment, a single-pulse, (15 ns) laser beam, aligned collinear with the z-axis, can be used to improve the photodissociation efficiency by irradiating these ions that are radially focused along the z-axis and within the confines of the laser beam. The length of the delay time between the dc pulse and the laser pulse is critical, as it controls the phase, and hence the spatial position, of the ions when the laser is fired. Under the given operating conditions, this method improves the photodissociation efficiency from 9% to 35%. The photodissociation efficiency steadily decreases with longer delay times as the oscillating ions undergo increasing numbers of collisions with the helium buffer gas and lose coherence. Since the secular frequencies of trapped ions are mass-dependent at a fixed rf amplitude, ions of different mass-to-charge ratios will cross the z-axis at different times. Mass-selective photodissociation is illustrated for a mixture of benzoyl-h(5) and -d(5) cations by appropriately adjusting the delay time between the dc pulse and the laser pulse. Simulations using the program ITSIM were used to design this experiment, and the data which describe the ion motion are provided.