A custom ion mobility mass spectrometer has been designed and built to investigate the photoisomerisation of molecular ions in the gas phase. The instrument differs from existing ion mobility mass spectrometers in that there is provision for optical access to the drift region to allow photo-excitation of the drifting ions as they drift. Photoisomerisation manifests as a change in the ions’ drift mobility, altering the time they take to drift through the instrument. By monitoring changes in the arrival time distribution as a function of laser wavelength it is possible to to collect photoisomerisation action spectra. The instrument has been used to probe the photoisomerisation of selected cationic carbocyanine dyes. Comparison of the dyes’ laser-off and laser-on arrival time distributions reveals that these dyes have a multitude of isomers, that can be interconverted using laser light. The peak absorptions of these dyes are shown to be blue-shifted relative to their absorption spectra in solution by 30-50nm. This work demonstrates that ion mobility can form the basis of a new spectroscopic technique for molecular ions that are not responsive to existing methods.