In this paper our aim is to investigate the properties of the modulation domain and more specifically, to evaluate the relative contributions of the modulation magnitude and phase spectra towards speech intelligibility. For this purpose, we extend the traditional (acoustic domain) analysis-modification-synthesis framework to include modulation domain processing. We use this framework to construct stimuli that retain only selected spectral components, for the purpose of objective and subjective intelligibility tests. We conduct three experiments. In the first, we investigate the relative contributions to intelligibility of the modulation magnitude, modulation phase, and acoustic phase spectra. In the second experiment, the effect of modulation frame duration on intelligibility for processing of the modulation magnitude spectrum is investigated. In the third experiment, the effect of modulation frame duration on intelligibility for processing of the modulation phase spectrum is investigated. Results of these experiments show that both the modulation magnitude and phase spectra are important for speech intelligibility, and that significant improvement is gained by the inclusion of acoustic phase information. They also show that smaller modulation frame durations improve intelligibility when processing the modulation magnitude spectrum, while longer frame durations improve intelligibility when processing the modulation phase spectrum.